Update doc on adding direct connection to Postgres

As Praefect now utilises a LISTEN functionality of the
Postgres database we should provide info on how to properly
configure a direct connection to the PostgreSQL database.

Part of: https://gitlab.com/gitlab-org/gitaly/-/issues/3334

doc/administration/gitaly/praefect.md

@@ -254,8 +254,23 @@ The database used by Praefect is now configured.
 
 To reduce PostgreSQL resource consumption, we recommend setting up and configuring
 [PgBouncer](https://www.pgbouncer.org/) in front of the PostgreSQL instance. To do
-this, replace value of the `POSTGRESQL_SERVER_ADDRESS` with corresponding IP or host
-address of the PgBouncer instance.
+this, set the corresponding IP or host address of the PgBouncer instance in
+`/etc/gitlab/gitlab.rb` by changing the following settings:
+
+- `praefect['database_host']`, for the address.
+- `praefect['database_port']`, for the port.
+
+Because PgBouncer manages resources more efficiently, Praefect still requires a
+direct connection to the PostgreSQL database because it uses
+[LISTEN](https://www.postgresql.org/docs/11/sql-listen.html)
+functionality that is [not supported](https://www.pgbouncer.org/features.html) by
+PgBouncer with `pool_mode = transaction`.
+
+Therefore, `praefect['database_host_no_proxy']` and `praefect['database_port_no_proxy']`
+should be set to a direct connection and not a PgBouncer connection.
+
+Save the changes to `/etc/gitlab/gitlab.rb` and
+[reconfigure Praefect](../restart_gitlab.md#omnibus-gitlab-reconfigure).
 
 This documentation doesn't provide PgBouncer installation instructions,
 but you can:
@@ -370,6 +385,8 @@ application server, or a Gitaly node.
    praefect['database_user'] = 'praefect'
    praefect['database_password'] = 'PRAEFECT_SQL_PASSWORD'
    praefect['database_dbname'] = 'praefect_production'
+   praefect['database_host_no_proxy'] = 'POSTGRESQL_SERVER_ADDRESS'
+   praefect['database_port_no_proxy'] = 5432
    ```
 
    If you want to use a TLS client certificate, the options below can be used: