media: v4l2-compat-ioctl32: better document the code

This file does a lot of non-trivial struff. Document it using
kernel-doc markups where needed and improve the comments inside
do_video_ioctl().

Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>