protocol: allow for a <description> element below <protocol>

The scanner parses this already, it doesn't do anything with it though.

The DTD requires the order to be copyright, description, then the interfaces.
That's largely a DTD limitation, the scanner doesn't care.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>