Documentation: Add sysfs ABI change for cfq's target latency.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/ABI/testing/sysfs-cfq-target-latency

+What:		/sys/block/<device>/iosched/target_latency
+Date:		March 2012
+contact:	Tao Ma <boyu.mt@taobao.com>
+Description:
+		The /sys/block/<device>/iosched/target_latency only exists
+		when the user sets cfq to /sys/block/<device>/scheduler.
+		It contains an estimated latency time for the cfq. cfq will
+		use it to calculate the time slice used for every task.