Document allowed pool names

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