media: pixfmt: Add H264_SLICE_RAW format documentation

The H264_SLICE_RAW format introduced before is meant for stateless
decoders that will need the H264 parsed slice data without the start code.

Let's document it.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>