QCoreApplication: add more information to processEvents() docs

Just shows how bad an idea to call this function it is.

Pick-to: 5.15
Fixes: QTBUG-88653
Change-Id: If51855da004b4f3fbf43fffd1648fc294065160d
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/kernel/qcoreapplication.cpp

@@ -1276,6 +1276,9 @@ void QCoreApplication::processEvents(QEventLoop::ProcessEventsFlags flags)
     \note Unlike the \l{QCoreApplication::processEvents(QEventLoop::ProcessEventsFlags flags)}{processEvents()}
     overload, this function also processes events that are posted while the function runs.
 
+    \note All events that were queued before the timeout will be processed,
+    however long it takes.
+
     \threadsafe
 
     \sa exec(), QTimer, QEventLoop::processEvents()