Document emission of commitDataRequest() signal in QApplication doc.

It is currently not clear that QApplication::commitData() emits
the commitDataRequest() signal. Users should know this if they plan
to override the default implementation.

Task-number: QTBUG-23117
Change-Id: I418af4b83795d53c9d86a2ac620c0f7c5bb9ada3
Reviewed-by: Geir Vattekar <geir.vattekar@nokia.com>

src/widgets/kernel/qapplication.cpp

@@ -2732,6 +2732,10 @@ QString QApplication::sessionKey() const
     all visible top-level widgets. If any event was rejected, the shutdown is
     canceled.
 
+    \note The default implementation emits the commitDataRequest() signal,
+    hence commitDataRequest() should be emitted when commitData() is
+    reimplemented.
+
     \sa isSessionRestored(), sessionId(), saveState(), {Session Management}
 */
 #ifndef QT_NO_SESSIONMANAGER