Clarify postgres exporter documentation

- Make it very clear that the localmachine address must be in the trust
  auth cidr list for postgres exporter to work.

Relates https://gitlab.com/gitlab-org/omnibus-gitlab/issues/3570Signed-off-by: Robert Marshall <rmarshall@gitlab.com>

doc/administration/monitoring/prometheus/postgres_exporter.md

@@ -16,6 +16,11 @@ To enable the postgres exporter:
    postgres_exporter['enable'] = true
    ```
 
+NOTE: **Note:**
+If PostgreSQL is configured on a separate node, make sure that the local
+address is [listed in `trust_auth_cidr_addresses`](../../high_availability/database.md#network-information) or the
+exporter will not be able to connect to the database.
+
 1. Save the file and [reconfigure GitLab][reconfigure] for the changes to
    take effect