[media] DocBook: add dvb_frontend.h to documentation

There are already some comments at dvb_frontend.h that are ready
for DocBook, although not properly formatted.

Convert them, and add this file to the device-drivers DocBook.

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