media: Documentation: Drop deprecated bytesused == 0

The V4L2 API historically allowed buffers to be queued with bytesused
set to 0 on output devices, in which case the driver would use the
buffer length. This behaviour is deprecated, and videobuf2 prints a
warning message in the kernel log. Drop it from the documentation.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>