Documentation/ABI/rnbd-clt: Add description for nr_poll_queues

describe how to set nr_poll_queues and enable the polling

Signed-off-by: Gioh Kim <gi-oh.kim@ionos.com>
Acked-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Link: https://lore.kernel.org/r/20210419073722.15351-15-gi-oh.kim@ionos.com


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