null_blk: add usage hints for no_sched

This parameter provide an option to disable io scheduler when nullb*
in multi-queue mode.
Signed-off-by: Nweiping zhang <zhangweiping@didichuxing.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

Documentation/block/null_blk.txt

@@ -73,3 +73,7 @@ use_per_node_hctx=[0/1]: Default: 0
 
 use_lightnvm=[0/1]: Default: 0
   Register device with LightNVM. Requires blk-mq and CONFIG_NVM to be enabled.
+
+no_sched=[0/1]: Default: 0
+  0: nullb* use default blk-mq io scheduler.
+  1: nullb* doesn't use io scheduler.