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>