Doc: mention that some filtered events need to be explicitly accepted

QKeyEvent's detailed description already documents this:

"A key event contains a special accept flag that indicates whether the
receiver will handle the key event. This flag is set by default for
QEvent::KeyPress and QEvent::KeyRelease, so there is no need to call
accept() when acting on a key event. For QEvent::ShortcutOverride the
receiver needs to explicitly accept the event to trigger the override.
Calling ignore() on a key event will propagate it to the parent widget.
The event is propagated up the parent widget chain until a widget
accepts it or an event filter consumes it."

However, someone looking at eventFilter() won't see this, resulting in
the shortcut not being consumed and propagating elsewhere, so mention
it in the docs for eventFilter() too.

Change-Id: I14fece52133be641ccdabd81f75706cd10b64669
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/corelib/kernel/qobject.cpp

@@ -1374,6 +1374,10 @@ void QObject::customEvent(QEvent * /* event */)
     might have reimplemented eventFilter() for its own internal
     purposes.
 
+    Some events, such as \l QEvent::ShortcutOverride must be explicitly
+    accepted (by calling \l {QEvent::}{accept()} on them) in order to prevent
+    propagation.
+
     \warning If you delete the receiver object in this function, be
     sure to return true. Otherwise, Qt will forward the event to the
     deleted object and the program might crash.