Update wxMenu::UpdateUI() parameter comment and documentation

The argument to this method should basically just never be used, so while we
still keep it for compatibility (because it doesn't cost anything to do it),
make it clear that it should never be specified in the new code and, also,
that this function is actually only used inside wxWidgets and shouldn't be
normally called from outside of the library at all.

include/wx/menu.h

@@ -275,9 +275,11 @@ public:
     // implementation helpers
     // ----------------------
 
-    // Updates the UI for a menu and all submenus recursively. source is the
-    // object that has the update event handlers defined for it. If NULL, the
-    // menu or associated window will be used.
+    // Updates the UI for a menu and all submenus recursively by generating
+    // wxEVT_UPDATE_UI for all the items.
+    //
+    // Do not use the "source" argument, it allows to override the event
+    // handler to use for these events, but this should never be needed.
     void UpdateUI(wxEvtHandler* source = NULL);
 
     // get the menu bar this menu is attached to (may be NULL, always NULL for