Fix uses of various qml doc commands

Signals documented with \qmlsignal should indicate handler name,
i.e. 'onSignal' rather than 'signal'.

Methods documented with \qmlmethod do not need to document 'void'
return values.

Also the name of any documented attribute should be qualified
with 'QtQuick2::<qmltype>'.

Change-Id: I206dd9e8f39c3b84e029ae9d4101b05d0bfb3478
Reviewed-by: Andrew den Exter <andrew.den-exter@nokia.com>