V4L/DVB (13572): v4l2-spec: Digital Video Timings API documentation

This patch updates the v4l2-dvb documentation for the new video timings API added.
Also updated the document based on comments from Hans Verkuil.

Signed-off-by: Muralidharan Karicheri <m-karicheri2@ti.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>