[media] DocBook/media/v4l: clarify the QUERYSTD documentation

Explicitly mention that this ioctl should return V4L2_STD_UNKNOWN if
not signal was detected.

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