Add back iostat -y or -w descriptions

The iostat -y and -w descriptions were left in cda0317,
get them back.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes openzfs#7479
Closes openzfs#7483

man/man8/zpool.8

@@ -24,10 +24,10 @@
 .\" Copyright 2016 Nexenta Systems, Inc.
 .\" Copyright (c) 2012 Cyril Plisko. All Rights Reserved.
 .\" Copyright (c) 2017 Datto Inc.
-.\" Copyright (c) 2017 George Melikov. All Rights Reserved.
+.\" Copyright (c) 2018 George Melikov. All Rights Reserved.
 .\" Copyright (c) 2017 Datto Inc.
 .\"
-.Dd June 28, 2017
+.Dd April 27, 2018
 .Dt ZPOOL 8 SMM
 .Os Linux
 .Sh NAME
@@ -1554,7 +1554,24 @@ block device.
 Verbose statistics Reports usage statistics for individual vdevs within the
 pool, in addition to the pool-wide statistics.
 .It Fl y
+Omit statistics since boot.
+Normally the first line of output reports the statistics since boot.
+This option suppresses that first line of output.
 .It Fl w
+Display latency histograms:
+.Pp
+.Ar total_wait :
+Total IO time (queuing + disk IO time).
+.Ar disk_wait :
+Disk IO time (time reading/writing the disk).
+.Ar syncq_wait :
+Amount of time IO spent in synchronous priority queues.  Does not include
+disk time.
+.Ar asyncq_wait :
+Amount of time IO spent in asynchronous priority queues.  Does not include
+disk time.
+.Ar scrub :
+Amount of time IO spent in scrub queue. Does not include disk time.
 .It Fl l
 Include average latency statistics:
 .Pp