Add documentation for the @rqw argument and change " - " into ": ".

Fixes: 84f60324 ("block: add rq_qos_wait to rq_qos") # v5.0-rc1~52^2~140.
Reviewed-by: NChaitanya Kulkarni <chiatanya.kulkarni@wdc.com>
Signed-off-by: NBart Van Assche <bvanassche@acm.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>