Done. Documentation follows.

The monitor detects if a server in the cluster is replicating from an external
master (a server that is not monitored by the monitor). If the replicating
server is the cluster master server, then the cluster itself is considered to
have an external master.

If a failover/switchover happens, the new master server is set to replicate from
the cluster external master server. The usename and password for the replication
are defined in `replication_user` and `replication_password`. The address and
port used are the ones shown by `SHOW ALL SLAVES STATUS` on the old cluster
master server. In the case of switchover, the old master also stops replicating
from the external server to preserve the topology.