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.

src/common/menucmn.cpp

@@ -593,9 +593,6 @@ wxMenuItem* wxMenuBase::FindItemByPosition(size_t position) const
 // wxMenu helpers used by derived classes
 // ----------------------------------------------------------------------------
 
-// Update 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.
 void wxMenuBase::UpdateUI(wxEvtHandler* source)
 {
     wxWindow * const win = GetWindow();
@@ -1095,7 +1092,7 @@ void wxMenuBarBase::UpdateMenus()
     {
         menu = GetMenu( n );
         if (menu != NULL)
-            menu->UpdateUI( NULL );
+            menu->UpdateUI();
     }
 }