Docs - add roles required for cloud PSQL

doc/administration/external_database.md

@@ -13,6 +13,11 @@ If you use a cloud-managed service, or provide your own PostgreSQL instance:
    [database requirements document](../install/requirements.md#database).
 1. Set up a `gitlab` username with a password of your choice. The `gitlab` user
    needs privileges to create the `gitlabhq_production` database.
+1. If you are using a cloud-managed service, you may need to grant additional
+   roles to your `gitlab` user:
+   - Amazon RDS requires the [`rds_superuser`](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Appendix.PostgreSQL.CommonDBATasks.html#Appendix.PostgreSQL.CommonDBATasks.Roles) role.
+   - Azure Database for PostgreSQL requires the [`azure_pg_admin`](https://docs.microsoft.com/en-us/azure/postgresql/howto-create-users#how-to-create-additional-admin-users-in-azure-database-for-postgresql) role.
+
 1. Configure the GitLab application servers with the appropriate connection details
    for your external PostgreSQL service in your `/etc/gitlab/gitlab.rb` file: