media: docs-rst: Clarify older field vs. first transmitted field

Add a paragraph to make it more clear that video equipment will transmit
fields in the same order the fields were captured, and replace some of
the "is transmitted first" language with "is the older field", since the
latter is the important info for motion compensation applications.

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>