Documents the protocol that user-space should follow when
communicating with stateless video decoders.

The stateless video decoding API makes use of the new request and tags
APIs. While it has been implemented with the Cedrus driver so far, it
should probably still be considered staging for a short while.
Signed-off-by: NAlexandre Courbot <acourbot@chromium.org>
Signed-off-by: NHans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>