make QMenuBar::trigger more clear about the ownership

On Mac, the QMenu added to QMenuBar has to be owned by the menu bar, in order to trigger the QMenuBar's triggered signal. (On linux, the signal is triggered even the menu is not the child of the menubar.)
In our document, it says "This signal is emitted when an action in a menu belonging to this menubar is triggered as a result of a mouse click; action is the action that caused the signal to be emitted." This doesn't clearly state the ownership requirement.