[media] dvb_frontend.h: document the struct dvb_frontend

That struct is used on every DVB Front End driver, as it
contains what's needed to register/use a frontend at the
Kernel.

Document it.

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