block: add documentation for io_timeout

Add documentation for /sys/block/<disk>/queue/io_timeout.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Weiping Zhang <zhangweiping@didiglobal.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/ABI/testing/sysfs-block

@@ -279,3 +279,12 @@ Description:
 		size in 512B sectors of the zones of the device, with
 		the eventual exception of the last zone of the device
 		which may be smaller.
+
+What:		/sys/block/<disk>/queue/io_timeout
+Date:		November 2018
+Contact:	Weiping Zhang <zhangweiping@didiglobal.com>
+Description:
+		io_timeout is the request timeout in milliseconds. If a request
+		does not complete in this time then the block driver timeout
+		handler is invoked. That timeout handler can decide to retry
+		the request, to fail it or to start a device recovery strategy.