V4L/DVB (3245): Added some comments about multiple tuner support.

- Added some comments to make clearer how to use ioctl api to handle
multiple tuners at the same board.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@brturbo.com.br>