[media] DocBook: media: add some general info about RF tuners

Add some info what is RF tuner in context of V4L RF tuner class.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>