Doc: document future direction of QCoreApplication::notify()

It will definitely not be called for events outside the main thread, but
we haven't decided for the main thread, in Qt 6.

[ChangeLog][Future direction notices] In Qt 6,
QCoreApplication::notify() will not be called for events being delivered
to objects outside the main thread. The reason for that is that the main
application object may begin destruction while those threads are still
delivering events, which is undefined behavior. Applications that
currently override notify() and use that function outside the main
thread are advised to find other solutions in the mean time.

Change-Id: I27eaacb532114dd188c4ffff13d5a5c8df3bc85b
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>