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: Ib4699ff82b8411b4cfb4400097deaca62f6d3bb0
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>