Clarify documentation about aboutToQuit being emitted with main loop running

After 449b45ff34 we emit aboutToQuit
before quitting the main event loop, matching the documentation and
semantics of "about to". Clarify this in the documentation, so that
client do not rely on a missing main event loop to trigger processing
of root level deleteLater() calls in a nested event loop.

Pick-to: 6.5 6.6
Change-Id: Ifd116394a1960814edf69f20c5aaff6a8e5cfc24
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/kernel/qcoreapplication.cpp

@@ -2161,6 +2161,12 @@ void QCoreApplicationPrivate::quit()
   last-second cleanup. Note that no user interaction is possible in
   this state.
 
+  \note At this point the main event loop is still running, but will
+  not process further events on return except QEvent::DeferredDelete
+  events for objects deleted via deleteLater(). If event processing is
+  needed, use a nested event loop or call QCoreApplication::processEvents()
+  manually.
+
   \sa quit()
 */