Add documentation for RF tuner bandwidth controls. These controls are
used to set filters on tuner signal path.

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