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

This is one of the most important functions of the DVB
frontend, containing the logic needed to set the parameters
at the demux and to send commands via SEC.

Document it.

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