[media] DocBook/media/v4l: update VIDIOC_DBG_ documentation

- Remove the "On failure the structure remains unchanged." part since
  that isn't necessarily true.
- Document the 'size' field.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>