[media] V4L2: Add documentation for SDI timings and related flags

Describe new needed constants defined by SDI format.

Signed-off-by: Charles-Antoine Couret <charles-antoine.couret@nexvision.fr>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>