Improve MariaDB Monitor documentation on auto_failover and auto_rejoin

Customer points out that passive mode is mentioned here:
https://mariadb.com/kb/en/mariadb-maxscale-2402-maxscale-2402-mariadb-monitor/#auto_failover
"For details on how MaxScale behaves in passive mode, see the documentation on failover_timeout below."
However, passive mode is not explained in failover timeout section. Perhaps it was an oversight or perhaps passive mode should be or is explained in a different place.