Doc: Document signals (not handlers) under \qmlsignal

Append the handler names to the end of the corresponding signal doc.

Task-number: QTBUG-35846
Change-Id: I325cdab75ef18a19c9f29d6333039c31baa8daf6
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

src/imports/multimedia/qdeclarativeradio.cpp

@@ -540,6 +540,8 @@ void QDeclarativeRadio::_q_availabilityChanged(QMultimedia::AvailabilityStatus a
 
     The \a frequency is returned in Hertz, and the \a stationId corresponds to the station Id
     in the \l RadioData for this radio station.
+
+    The corresponding handler is \c onStationFound.
   */
 
 QT_END_NAMESPACE