[media] Docbook: Document struct analog_parameters

That struct inside dvb-frontend.h stores some parameters
from V4L2 API (videodev2.h), in order to be used by the
hybrid analog/digital TV tuners.

Document it.

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