zfs-allow.8: mention 'bookmark' permission

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Lauri Tirkkonen <lauri@hacktheplanet.fi>
Closes openzfs#12064

man/man8/zfs-allow.8

@@ -195,6 +195,7 @@ The following permissions are available:
 NAME             TYPE           NOTES
 allow            subcommand     Must also have the permission that is
                                 being allowed
+bookmark         subcommand
 clone            subcommand     Must also have the 'create' ability and
                                 'mount' ability in the origin file system
 create           subcommand     Must also have the 'mount' ability.