Clarify wxGetTopLevelParent() behaviour for TLWs.

It doesn't return the parent of a TLW but TLW itself. Whether this is a good
idea or bad one, this is how it works since a very long time and we don't want
to change it any more.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75357 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/window.h

@@ -3789,6 +3789,8 @@ wxWindow* wxGetActiveWindow();
     Returns the first top level parent of the given window, or in other words,
     the frame or dialog containing it, or @NULL.
 
+    Notice that if @a window is itself already a TLW, it is returned directly.
+
     @header{wx/window.h}
 */
 wxWindow* wxGetTopLevelParent(wxWindow* window);