Document PgBouncer session pooling for Praefect

doc/administration/gitaly/index.md

@@ -175,7 +175,7 @@ Gitaly Cluster consists of multiple components:
 - [Load balancer](praefect.md#load-balancer) for distributing requests and providing fault-tolerant access to
   Praefect nodes.
 - [Praefect](praefect.md#praefect) nodes for managing the cluster and routing requests to Gitaly nodes.
-- [PostgreSQL database](praefect.md#postgresql) for persisting cluster metadata and [PgBouncer](praefect.md#pgbouncer),
+- [PostgreSQL database](praefect.md#postgresql) for persisting cluster metadata and [PgBouncer](praefect.md#use-pgbouncer),
   recommended for pooling Praefect's database connections.
 - Gitaly nodes to provide repository storage and Git access.
 

doc/administration/postgresql/pgbouncer.md

@@ -52,6 +52,20 @@ This content has been moved to a [new location](replication_and_failover.md#conf
    }
    ```
 
+   You can pass additional configuration parameters per database, for example:
+
+   ```ruby
+   pgbouncer['databases'] = {
+     gitlabhq_production: {
+        ...
+        pool_mode: 'transaction'
+     }
+   }
+   ```
+
+   Use these parameters with caution. For the complete list of parameters refer to the
+   [PgBouncer documentation](https://www.pgbouncer.org/config.html#section-databases).
+
 1. Run `gitlab-ctl reconfigure`
 
 1. On the node running Puma, make sure the following is set in `/etc/gitlab/gitlab.rb`