docs: sysfs-block-device: improve ncq_prio_enable documentation

NCQ priority is an optional feature of the NCQ feature set and should
not be confused with the NCQ feature set itself. Clarify the
description of the ncq_prio_enable attribute to avoid this confusion.

Also add the missing documentation for the equivalent
sas_ncq_prio_enable attribute.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20210816014456.2191776-11-damien.lemoal@wdc.com


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