media: pixfmt: Document the HEVC slice pixel format

Document the current state of the HEVC slice pixel format.
The format will need to evolve in the future, which is why it is
not part of the public API.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>