Add @onlyfor tag for wxSound::IsPlaying() documentation.

This is not totally precise as the method is available in all Unix-based ports
but currently there doesn't seem to be any way to specify this so just mention
its availability in wxGTK which is what counts.

Closes #14002.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70643 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/sound.h

@@ -72,7 +72,12 @@ public:
 
     /**
         Returns @true if a sound is played at the moment.
-        This method is currently not implemented under Windows.
+
+        This method is currently not available under Windows and may not be
+        always implemented in Unix ports depending on the compilation options
+        used (in this case it just always returns @false).
+
+        @onlyfor{wxgtk,wxosx}
     */
     static bool IsPlaying();