Document disallowed copy and allowed move better.

Moreover when the deleted function was private, the compilation failed
with (rather cryptic) error that the function is private instead of
noticing that the function is deleted.

src/AbstractTexture.h

@@ -95,9 +95,6 @@ do nothing.
 class MAGNUM_EXPORT AbstractTexture {
     friend class Context;
 
-    AbstractTexture(const AbstractTexture&) = delete;
-    AbstractTexture& operator=(const AbstractTexture&) = delete;
-
     public:
         /**
          * @brief Max supported layer count
@@ -124,9 +121,15 @@ class MAGNUM_EXPORT AbstractTexture {
          */
         virtual ~AbstractTexture() = 0;
 
+        /** @brief Copying is not allowed */
+        AbstractTexture(const AbstractTexture&) = delete;
+
         /** @brief Move constructor */
         AbstractTexture(AbstractTexture&& other);
 
+        /** @brief Copying is not allowed */
+        AbstractTexture& operator=(const AbstractTexture&) = delete;
+
         /** @brief Move assignment */
         AbstractTexture& operator=(AbstractTexture&& other);