Document that using an external Postgres server has serious downsides

Related to:

- https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1682
- https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1679

docs/configuring-playbook-external-postgres.md

@@ -5,7 +5,9 @@ If that's alright, you can skip this.
 
 If you'd like to use an external PostgreSQL server that you manage, you can edit your configuration file  (`inventory/host_vars/matrix.<your-domain>/vars.yml`).
 
-It should be something like this:
+**NOTE**: using **an external Postgres server is currently [not very seamless](https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1682#issuecomment-1061461683) when it comes to enabling various other playbook services** - you will need to create a new database/credentials for each service and to point each service to its corresponding database using custom `vars.yml` configuration. **For the best experience with the playbook, stick to using the integrated Postgres server**.
+
+If you'd like to use an external Postgres server, use a custom `vars.yml` configuration like this:
 
 ```yaml
 matrix_postgres_enabled: false
@@ -15,6 +17,10 @@ matrix_synapse_database_host: "your-postgres-server-hostname"
 matrix_synapse_database_user: "your-postgres-server-username"
 matrix_synapse_database_password: "your-postgres-server-password"
 matrix_synapse_database_database: "your-postgres-server-database-name"
+
+# Rewire any other service (each `matrix-*` role) you may wish to use to use your external Postgres server.
+# Each service expects to have its own dedicated database on the Postgres server
+# and uses its own variable names (see `roles/matrix-*/defaults/main.yml) for configuring Postgres connectivity.
 ```
 
 The database (as specified in `matrix_synapse_database_database`) must exist and be accessible with the given credentials.

roles/matrix-postgres/defaults/main.yml

@@ -1,5 +1,8 @@
 ---
 
+# Controls if the Postgres server managed by the playbook is enabled.
+# You can turn it off and use an external Postgres server by setting this to `false`.
+# Doing this has various downsides. See `docs/configuring-playbook-external-postgres.md` to learn more.
 matrix_postgres_enabled: true
 
 matrix_postgres_connection_hostname: "matrix-postgres"