Change the handler documentation to signal documentation

The signal handlers need to be converted to signal documentation.

1) Do a grep for \qmlsignal.
2) Document the signal, not the handler. Document 'clicked' not 'onClicked'

Some style guidelines:

"This signal is emitted when ..." #note the conditions for the signal being emitted. Who emits the signal is not necessarily mentioned
"The corresponding handler is ..." #mention the signal handler