Add the Xr's to the SEE ALSO as well

There are a ton of zfs-* and zpool-* man pages. This adds them to
the SEE ALSO section so that people can more quickly look through
what all the options are, now that the pages have been split.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: John-Mark Gurney <jmg@funkthat.com>
Closes #10589