Documentation: block: ioprio: Update schedulers

This doc hasn't been touched in a while, in the meantime some
new io schedulers were added (e.g. all of mq), some with ioprio
support.

Also reword the introduction to remove reference to CFQ and the
limitation that io priorities only work on reads, which is no longer
true.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/a86cfdc8-016f-40f1-8b58-0cb15d2a792c@arm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>