Trade: document default implementation of virtual functions.

Better to have these documented, I came to this after longer pause and
thought that these were all pure virtual.

src/Magnum/Trade/AbstractImporter.h

@@ -496,148 +496,296 @@ class MAGNUM_EXPORT AbstractImporter: public PluginManager::AbstractManagingPlug
         /** @brief Implementation for @ref close() */
         virtual void doClose() = 0;
 
-        /** @brief Implementation for @ref defaultScene() */
+        /**
+         * @brief Implementation for @ref defaultScene()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doDefaultScene();
 
-        /** @brief Implementation for @ref sceneCount() */
+        /**
+         * @brief Implementation for @ref sceneCount()
+         *
+         * Default implementation returns `0`.
+         */
         virtual UnsignedInt doSceneCount() const;
 
-        /** @brief Implementation for @ref sceneForName() */
+        /**
+         * @brief Implementation for @ref sceneForName()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doSceneForName(const std::string& name);
 
-        /** @brief Implementation for @ref sceneName() */
+        /**
+         * @brief Implementation for @ref sceneName()
+         *
+         * Default implementation returns empty string.
+         */
         virtual std::string doSceneName(UnsignedInt id);
 
         /** @brief Implementation for @ref scene() */
         virtual std::optional<SceneData> doScene(UnsignedInt id);
 
-        /** @brief Implementation for @ref lightCount() */
+        /**
+         * @brief Implementation for @ref lightCount()
+         *
+         * Default implementation returns `0`.
+         */
         virtual UnsignedInt doLightCount() const;
 
-        /** @brief Implementation for @ref lightForName() */
+        /**
+         * @brief Implementation for @ref lightForName()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doLightForName(const std::string& name);
 
-        /** @brief Implementation for @ref lightName() */
+        /**
+         * @brief Implementation for @ref lightName()
+         *
+         * Default implementation returns empty string.
+         */
         virtual std::string doLightName(UnsignedInt id);
 
         /** @brief Implementation for @ref light() */
         virtual std::optional<LightData> doLight(UnsignedInt id);
 
-        /** @brief Implementation for @ref cameraCount() */
+        /**
+         * @brief Implementation for @ref cameraCount()
+         *
+         * Default implementation returns `0`.
+         */
         virtual UnsignedInt doCameraCount() const;
 
-        /** @brief Implementation for @ref cameraForName() */
+        /**
+         * @brief Implementation for @ref cameraForName()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doCameraForName(const std::string& name);
 
-        /** @brief Implementation for @ref cameraName() */
+        /**
+         * @brief Implementation for @ref cameraName()
+         *
+         * Default implementation returns empty string.
+         */
         virtual std::string doCameraName(UnsignedInt id);
 
         /** @brief Implementation for @ref camera() */
         virtual std::optional<CameraData> doCamera(UnsignedInt id);
 
-        /** @brief Implementation for @ref object2DCount() */
+        /**
+         * @brief Implementation for @ref object2DCount()
+         *
+         * Default implementation returns `0`.
+         */
         virtual UnsignedInt doObject2DCount() const;
 
-        /** @brief Implementation for @ref object2DForName() */
+        /**
+         * @brief Implementation for @ref object2DForName()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doObject2DForName(const std::string& name);
 
-        /** @brief Implementation for @ref object2DName() */
+        /**
+         * @brief Implementation for @ref object2DName()
+         *
+         * Default implementation returns empty string.
+         */
         virtual std::string doObject2DName(UnsignedInt id);
 
         /** @brief Implementation for @ref object2D() */
         virtual std::unique_ptr<ObjectData2D> doObject2D(UnsignedInt id);
 
-        /** @brief Implementation for @ref object3DCount() */
+        /**
+         * @brief Implementation for @ref object3DCount()
+         *
+         * Default implementation returns `0`.
+         */
         virtual UnsignedInt doObject3DCount() const;
 
-        /** @brief Implementation for @ref object3DForName() */
+        /**
+         * @brief Implementation for @ref object3DForName()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doObject3DForName(const std::string& name);
 
-        /** @brief Implementation for @ref object3DName() */
+        /**
+         * @brief Implementation for @ref object3DName()
+         *
+         * Default implementation returns empty string.
+         */
         virtual std::string doObject3DName(UnsignedInt id);
 
         /** @brief Implementation for @ref object3D() */
         virtual std::unique_ptr<ObjectData3D> doObject3D(UnsignedInt id);
 
-        /** @brief Implementation for @ref mesh2DCount() */
+        /**
+         * @brief Implementation for @ref mesh2DCount()
+         *
+         * Default implementation returns `0`.
+         */
         virtual UnsignedInt doMesh2DCount() const;
 
-        /** @brief Implementation for @ref mesh2DForName() */
+        /**
+         * @brief Implementation for @ref mesh2DForName()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doMesh2DForName(const std::string& name);
 
-        /** @brief Implementation for @ref mesh2DName() */
+        /**
+         * @brief Implementation for @ref mesh2DName()
+         *
+         * Default implementation returns empty string.
+         */
         virtual std::string doMesh2DName(UnsignedInt id);
 
         /** @brief Implementation for @ref mesh2D() */
         virtual std::optional<MeshData2D> doMesh2D(UnsignedInt id);
 
-        /** @brief Implementation for @ref mesh3DCount() */
+        /**
+         * @brief Implementation for @ref mesh3DCount()
+         *
+         * Default implementation returns `0`.
+         */
         virtual UnsignedInt doMesh3DCount() const;
 
-        /** @brief Implementation for @ref mesh3DForName() */
+        /**
+         * @brief Implementation for @ref mesh3DForName()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doMesh3DForName(const std::string& name);
 
-        /** @brief Implementation for @ref mesh3DName() */
+        /**
+         * @brief Implementation for @ref mesh3DName()
+         *
+         * Default implementation returns empty string.
+         */
         virtual std::string doMesh3DName(UnsignedInt id);
 
         /** @brief Implementation for @ref mesh3D() */
         virtual std::optional<MeshData3D> doMesh3D(UnsignedInt id);
 
-        /** @brief Implementation for @ref materialCount() */
+        /**
+         * @brief Implementation for @ref materialCount()
+         *
+         * Default implementation returns `0`.
+         */
         virtual UnsignedInt doMaterialCount() const;
 
-        /** @brief Implementation for @ref materialForName() */
+        /**
+         * @brief Implementation for @ref materialForName()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doMaterialForName(const std::string& name);
 
-        /** @brief Implementation for @ref materialName() */
+        /**
+         * @brief Implementation for @ref materialName()
+         *
+         * Default implementation returns empty string.
+         */
         virtual std::string doMaterialName(UnsignedInt id);
 
         /** @brief Implementation for @ref material() */
         virtual std::unique_ptr<AbstractMaterialData> doMaterial(UnsignedInt id);
 
-        /** @brief Implementation for @ref textureCount() */
+        /**
+         * @brief Implementation for @ref textureCount()
+         *
+         * Default implementation returns `0`.
+         */
         virtual UnsignedInt doTextureCount() const;
 
-        /** @brief Implementation for @ref textureForName() */
+        /**
+         * @brief Implementation for @ref textureForName()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doTextureForName(const std::string& name);
 
-        /** @brief Implementation for @ref textureName() */
+        /**
+         * @brief Implementation for @ref textureName()
+         *
+         * Default implementation returns empty string.
+         */
         virtual std::string doTextureName(UnsignedInt id);
 
         /** @brief Implementation for @ref texture() */
         virtual std::optional<TextureData> doTexture(UnsignedInt id);
 
-        /** @brief Implementation for @ref image1DCount() */
+        /**
+         * @brief Implementation for @ref image1DCount()
+         *
+         * Default implementation returns `0`.
+         */
         virtual UnsignedInt doImage1DCount() const;
 
-        /** @brief Implementation for @ref image1DForName() */
+        /**
+         * @brief Implementation for @ref image1DForName()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doImage1DForName(const std::string& name);
 
-        /** @brief Implementation for @ref image1DName() */
+        /**
+         * @brief Implementation for @ref image1DName()
+         *
+         * Default implementation returns empty string.
+         */
         virtual std::string doImage1DName(UnsignedInt id);
 
         /** @brief Implementation for @ref image1D() */
         virtual std::optional<ImageData1D> doImage1D(UnsignedInt id);
 
-        /** @brief Implementation for @ref image2DCount() */
+        /**
+         * @brief Implementation for @ref image2DCount()
+         *
+         * Default implementation returns `0`.
+         */
         virtual UnsignedInt doImage2DCount() const;
 
-        /** @brief Implementation for @ref image2DForName() */
+        /**
+         * @brief Implementation for @ref image2DForName()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doImage2DForName(const std::string& name);
 
-        /** @brief Implementation for @ref image2DName() */
+        /**
+         * @brief Implementation for @ref image2DName()
+         *
+         * Default implementation returns empty string.
+         */
         virtual std::string doImage2DName(UnsignedInt id);
 
         /** @brief Implementation for @ref image2D() */
         virtual std::optional<ImageData2D> doImage2D(UnsignedInt id);
 
-        /** @brief Implementation for @ref image3DCount() */
+        /**
+         * @brief Implementation for @ref image3DCount()
+         *
+         * Default implementation returns `0`.
+         */
         virtual UnsignedInt doImage3DCount() const;
 
-        /** @brief Implementation for @ref image3DForName() */
+        /**
+         * @brief Implementation for @ref image3DForName()
+         *
+         * Default implementation returns `-1`.
+         */
         virtual Int doImage3DForName(const std::string& name);
 
-        /** @brief Implementation for @ref image3DName() */
+        /**
+         * @brief Implementation for @ref image3DName()
+         *
+         * Default implementation returns empty string.
+         */
         virtual std::string doImage3DName(UnsignedInt id);
 
         /** @brief Implementation for @ref image3D() */