Document that wxClipboard::Flush() is not implemented in wxGTK.

See #10515.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71542 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/clipbrd.h

@@ -102,6 +102,9 @@ public:
         (possibly eating memory), otherwise the clipboard will be emptied on
         exit.
 
+        Currently this method is not implemented in X11-based ports, i.e.
+        wxGTK, wxX11 and wxMotif and always returns @false there.
+
         @return @false if the operation is unsuccessful for any reason.
     */
     virtual bool Flush();