Mention wxGTK wxAnimation implementation limitations in the docs.

The native GTK implementation doesn't provide information about the frames
count nor access to individual frames.

See #13365.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68437 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/animate.h

@@ -228,11 +228,17 @@ public:
 
     /**
         Returns the i-th frame as a wxImage.
+
+        This method is not implemented in the native wxGTK implementation of
+        this class and always returns an invalid image there.
     */
     virtual wxImage GetFrame(unsigned int i) const;
 
     /**
         Returns the number of frames for this animation.
+
+        This method is not implemented in the native wxGTK implementation of
+        this class and always returns 0 there.
     */
     virtual unsigned int GetFrameCount() const;