block: update documentation for deadline fifo_batch tunable

Similar to read_expire mentioned above, but for writes.

fifo_batch

fifo_batch	(number of requests)

----------

When a read request expires its deadline, we must move some requests from

the sorted io scheduler list to the block device dispatch queue. fifo_batch

controls how many requests we move.

Requests are grouped into ``batches'' of a particular data direction (read or

write) which are serviced in increasing sector order.  To limit extra seeking,

deadline expiries are only checked between batches.  fifo_batch controls the

maximum number of requests per batch.

This parameter tunes the balance between per-request latency and aggregate

throughput.  When low latency is the primary concern, smaller is better (where

a value of 1 yields first-come first-served behaviour).  Increasing fifo_batch

generally improves throughput, at the cost of latency variation.

writes_starved	(number of dispatches)