[media] DocBook media: clarify v4l2_pix_format and v4l2_pix_format_mplane fields

Be more specific with regards to how some of these fields are interpreted.
In particular the height value and which fields can be set by the application.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>