Doc: Document signals (not handlers) under \qmlattachedsignal

- They're not "attached properties" either.
- Append the handler names to the end of the corresponding signal doc.
- Update descriptions and links

Task-number: QTBUG-35846
Change-Id: I54e93187b3209546ec344a20e0482c98d7f14109
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>