block: improve ioprio class description comment

In include/usapi/linux/ioprio.h, change the ioprio class enum comment
to remove the outdated reference to CFQ and mention BFQ and mq-deadline
instead. Also document the high priority NCQ command use for RT class
IOs directed at ATA drives that support NCQ priority.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Link: https://lore.kernel.org/r/20210811033702.368488-3-damien.lemoal@wdc.com


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