zfs-list.8: clarify listing snapshots
Clarify how to include snapshots in the zpool list output by
referencing the full name of the listsnapshots pool property,
and the zpool list -t snapshot option.
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11562
Closes #11565