Update docs/configuring-playbook-prometheus-grafana.md: move variables for setting username and password of Postgres exporter to the YAML block above

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-prometheus-grafana.md

@@ -40,16 +40,24 @@ Expanding on the metrics exposed by the Synapse exporter and the Node exporter,
 
 To enable it, add the following configuration to your `vars.yml` file:
 
+**Note**: `prometheus_postgres_exporter_database_username` has nothing to do with your Matrix user ID. It can be any string you'd like.
+
 ```yaml
 prometheus_postgres_exporter_enabled: true
+
+# The username for the user that the exporter uses to connect to the database.
+# Uncomment and adjust this part if you'd like to use a username different than the default.
+# prometheus_postgres_exporter_database_username: "matrix_prometheus_postgres_exporter"
+
+# The password for the user that the exporter uses to connect to the database. By default, this is auto-generated by the playbook.
+# Uncomment and adjust this part if you'd like to set the password by yourself.
+# prometheus_postgres_exporter_database_password: "PASSWORD_HERE"
 ```
 
 Name | Description
 -----|----------
 `prometheus_postgres_exporter_enabled`|Enable the postgres prometheus exporter. This sets up the docker container, connects it to the database and adds a 'job' to the prometheus config which tells prometheus about this new exporter. The default is 'false'
-`prometheus_postgres_exporter_database_username`| The 'username' for the user that the exporter uses to connect to the database. The default is 'matrix_prometheus_postgres_exporter'
-`prometheus_postgres_exporter_database_password`| The 'password' for the user that the exporter uses to connect to the database. By default, this is auto-generated by the playbook
-`prometheus_postgres_exporter_container_labels_traefik_enabled`|If set to `true`, exposes the Postgres exporter metrics on `https://matrix.example.com/metrics/postgres-exporter` for usage with an [external Prometheus server](configuring-playbook-prometheus-grafana.md#collecting-metrics-to-an-external-prometheus-server). To password-protect the metrics, see `matrix_metrics_exposure_http_basic_auth_users` on that other documentation page.
+`prometheus_postgres_exporter_container_labels_traefik_enabled`|If set to `true`, exposes the Postgres exporter metrics on `https://matrix.example.com/metrics/postgres-exporter` for usage with an [external Prometheus server](#collecting-metrics-to-an-external-prometheus-server). To password-protect the metrics, see `matrix_metrics_exposure_http_basic_auth_users` below.
 
 #### Extending the configuration