docs: update cmdprio_percentage documentation

docs: update cmdprio_percentage documentation

Commit 1437d6357429 ("libaio,io_uring: relax cmdprio_percentage
constraints") relaxed the cmdprio_percentage constraints such that
cmdprio_percentage and prioclass/prio could be used together.

However, it forgot to remove the mention of this constraint from
the docs. Update the docs to reflect the new behavior.

Fixes: 1437d6357429 ("libaio,io_uring: relax cmdprio_percentage constraints")
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20211112095428.158300-2-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>