Docs - disable bundled PSQL on secondary node setup

doc/administration/geo/replication/external_database.md

@@ -133,6 +133,10 @@ To configure the connection to the external read-replica database and enable Log
 
    gitlab_rails['db_username'] = 'gitlab'
    gitlab_rails['db_host'] = '<database_read_replica_host>'
+
+   # Disable the bundled Omnibus PostgreSQL, since we are
+   # using an external PostgreSQL
+   postgresql['enable'] = false
    ```
 
 1. Save the file and [reconfigure GitLab](../../restart_gitlab.md#omnibus-gitlab-reconfigure)