QmlDesigner: Improve tool tips of the snap actions second try

Change-Id: I5efed5ffdccc5ce0aa13b1e7296137396ca28efe
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Doc: snapping to anchors in the Qt Quick Designer

Add documentation for the new button and update
the screen shots. Fix changed field names.

Change-Id: I291f63b4c87090012a3f4bf2cbbc4172e3b58aa5
Reviewed-by: Marco Bubke <marco.bubke@digia.com>