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.

interface/wx/menu.h

@@ -1036,11 +1036,13 @@ public:
     virtual void SetTitle(const wxString& title);
 
     /**
-        Sends events to @a source (or owning window if @NULL) to update the
-        menu UI.
+        Update the state of all menu items, recursively, by generating @c
+        wxEVT_UPDATE_UI events for them.
 
-        This is called just before the menu is popped up with wxWindow::PopupMenu,
-        but the application may call it at other times if required.
+        This is an internal wxWidgets function and shouldn't normally be called
+        from outside of the library. If it is called, @a source argument should
+        not be used, it is deprecated and exists only for backwards
+        compatibility.
     */
     void UpdateUI(wxEvtHandler* source = NULL);