docs: sysfs-block-device: document ncq_prio_supported

Add documentation for the new device attribute file ncq_prio_supported,
and its SAS HBA equivalent sas_ncq_prio_supported.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20210816014456.2191776-12-damien.lemoal@wdc.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>