Document the fact that the quit shortcut may skip the emission of the QQuickWindow::closing() signal when using a "global" menu bar

As this answer explains, if there is no menu item whose text is "quit" or "exit", one will be created, and it will call QCoreApplication::quit(). This skips the emission of the QQuickWindow::closing() signal, which means code like this will be skipped:

    onClosing: {
        close.accepted = false
        doIfChangesDiscarded(function() { Qt.quit() })
    }

The documentation for http://doc.qt.io/qt-5/qquickwindow.html#closing should mention this (and link to http://doc.qt.io/qt-5/qmenubar.html#qmenubar-as-a-global-menu-bar), as it is very confusing to have the behaviour be different across platforms and not know why.