[media] DocBook: document RF tuner gain controls

Add documentation for LNA, mixer and IF gain controls. These
controls are RF tuner specific.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>