Document allowed pool names

PR #7208 was a patch to allow non-reserved pool names which begin with
mirror, raidz, spare (but do not equal), however we'd rather document
it in the man page for compatibility with other OpenZFS implementations,
to avoid pool names that may not work on non-Linux platforms.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #7216

man/man8/zpool.8

@@ -27,7 +27,7 @@
 .\" Copyright 2017 Nexenta Systems, Inc.
 .\" Copyright (c) 2017 Open-E, Inc. All Rights Reserved.
 .\"
-.Dd January 10, 2018
+.Dd March 9, 2018
 .Dt ZPOOL 8 SMM
 .Os Linux
 .Sh NAME
@@ -888,7 +888,11 @@ The pool names
 .Sy spare
 and
 .Sy log
-are reserved, as are names beginning with the pattern
+are reserved, as are names beginning with
+.Sy mirror ,
+.Sy raidz ,
+.Sy spare ,
+and the pattern
 .Sy c[0-9] .
 The
 .Ar vdev