Document that images in buttons and menus might not be shown under GTK.

Mention gtk-{button,menu}-images GTK+ settings.

See #11369.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63609 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/menuitem.h

@@ -314,6 +314,11 @@ public:
         @a checked is @true (default value) or SetBitmaps(wxNullBitmap, bmp)
         otherwise.
 
+        Notice that GTK+ uses a global setting called @c gtk-menu-images to
+        determine if the images should be shown in the menus at all. If it is
+        off (which is the case in e.g. Gnome 2.28 by default), no images will
+        be shown, consistently with the native behaviour.
+
         @onlyfor{wxmsw,wxosx,wxgtk}
     */
     virtual void SetBitmap(const wxBitmap& bmp, bool checked = true);