Geo: Document how to use `gitlab-ctl set-replication-password` on external PostgreSQL
gitlab-ctl set-replication-password
only works on an omnibus PostgreSQL node, and it won't work if you run it on a Rails host in an HA environment. If you are using an external PostgreSQL instance (e.g. Cloud SQL), you need to know to run:
ALTER USER gitlab_replicator WITH ENCRYPTED PASSWORD 'MY-PASSWORD-HERE'