V4L/DVB: v4l spec: document what odd and even fields are

Document that odd == top and even == bottom. I can never remember this,
so it is probably worth documenting.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>