Mention that wxOSX doesn't implement SetLayoutDirection()

At least avoid wasting people time with debugging why it doesn't work.

interface/wx/window.h

@@ -3051,6 +3051,9 @@ public:
 
     /**
         Sets the layout direction for this window.
+
+        This function is only supported under MSW and GTK platforms, but not
+        under Mac currently.
     */
     virtual void SetLayoutDirection(wxLayoutDirection dir);