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

Documentation/ABI/testing/sysfs-block-device

@@ -55,6 +55,20 @@ Date:		Oct, 2016
 KernelVersion:	v4.10
 Contact:	linux-ide@vger.kernel.org
 Description:
-		(RW) Write to the file to turn on or off the SATA ncq (native
-		command queueing) support. By default this feature is turned
-		off.
+		(RW) Write to the file to turn on or off the SATA NCQ (native
+		command queueing) priority support. By default this feature is
+		turned off. If the device does not support the SATA NCQ
+		priority feature, writing "1" to this file results in an error.
+
+
+What:		/sys/block/*/device/sas_ncq_prio_enable
+Date:		Oct, 2016
+KernelVersion:	v4.10
+Contact:	linux-ide@vger.kernel.org
+Description:
+		(RW) This is the equivalent of the ncq_prio_enable attribute
+		file for SATA devices connected to a SAS host-bus-adapter
+		(HBA) implementing support for the SATA NCQ priority feature.
+		This file does not exist if the HBA driver does not implement
+		support for the SATA NCQ priority feature, regardless of the
+		device support for this feature.