media: uapi: h264: Add documentation to the interface header

In preparation for making the interface public,
document all the structures. Special care is taken to
annotate those fields that depart from the H264 syntax.

This commit only adds documentation and doesn't affect
functionality in any way.
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>