[media] DocBook media: improve V4L2_DV_FL_HALF_LINE documentation

Explicitly specify where the half-line is added or removed in
each field.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Prashant Laddha <prladdha@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>