Added some text and a reference to the consistency testing chapter.
This follows a discussion started by Edward d'Auvergne at:
https://mail.gna.org/public/relax-devel/2012-09/msg00019.html
(Message-id: <CAED9pY8XhmmvyfBS7mZA8XZ=4mJZe9TuGjARHoV2tVcjjV9SrQ@...>)
