blk-mq: fix stale nr_requests documentation

The nr_requests documentation is still the removed single queue, remove
it and update to current blk-mq.

Signed-off-by: Yu Kuai <[e-mail]>
Reviewed-by: Nilay Shroff <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

Documentation/ABI/stable/sysfs-block

···
 Contact:	[e-mail]
 Description:
 		[RW] This controls how many requests may be allocated in the
-		block layer for read or write requests. Note that the total
-		allocated number may be twice this amount, since it applies only
-		to reads or writes (not the accumulated sum).
-
-		To avoid priority inversion through request starvation, a
-		request queue maintains a separate request pool per each cgroup
-		when CONFIG_BLK_CGROUP is enabled, and this parameter applies to
-		each such per-block-cgroup request pool.  IOW, if there are N
-		block cgroups, each request queue may have up to N request
-		pools, each independently regulated by nr_requests.
+		block layer. Noted this value only represents the quantity for a
+		single blk_mq_tags instance. The actual number for the entire
+		device depends on the hardware queue count, whether elevator is
+		enabled, and whether tags are shared.
 
 
 What:		/sys/block/<disk>/queue/nr_zones