[media] DocBook: clarify that sequence is also set for output devices

It was not entirely obvious that the sequence count should also
be set for output devices. Also made it more explicit that this
sequence counter counts frames, not fields.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>