[media] dvb-frontend.h: document struct dtv_frontend_properties

Most of the parameters here are already well defined at
the userspace documentation. Yet, it is good to add some
documentation, for the developers to be sure that they
are the same as the ones at userspace API.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>