[media] DocBook media: update three sections of common.xml

Updates for the "Tuners and Modulators", "Video Standards" and
"Digital Video (DV) Timings" sections.

Besides lots of trivial little fixes the main changes are:

- Remove two footnotes from "Video Standards": the first is a discussion
  of alternative methods of setting standards, which is pretty pointless
  since the standards API is effectively frozen anyway, and the second
  points to 'rationale' that makes little or no sense to me.

- Clarify a few things in the "Digital Video (DV) Timings" section.
  It was awkwardly formatted as well: there used to be a list with
  multiple bullets that has been reduced to a single item, so drop the
  list and rewrite that text.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>