Trade: doc++

Not bothering with all the engrish there, this thing needs an overhaul
anyway.

src/Magnum/Trade/AbstractImageConverter.h

@@ -134,6 +134,11 @@ checked by the implementation:
     @ref ImageConverterFeature::ConvertData is supported.
 -   The function @ref doExportToData(const CompressedImageView2D&) is called
     only if @ref ImageConverterFeature::ConvertCompressedData is supported.
+-   The function @ref doExportToFile(const ImageView2D&, const std::string&) is
+    called only if @ref ImageConverterFeature::ConvertFile is supported.
+-   The function @ref doExportToFile(const CompressedImageView2D&, const std::string&)
+    is called only if @ref ImageConverterFeature::ConvertCompressedFile is
+    supported.
 
 @m_class{m-block m-warning}