Documentation: media: improve V4L2_CID_MIN_BUFFERS_FOR_*, doc

Clearly state that the V4L2_CID_MIN_BUFFERS_FOR_OUTPUT and
V4L2_CID_MIN_BUFFERS_FOR_CAPTURE controls are required for
stateful codecs.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>