Docs - add roles required for cloud PSQL geo tracking db

doc/administration/geo/replication/external_database.md

@@ -156,6 +156,13 @@ potential replication issues. Omnibus automatically configures a tracking databa
 when `roles ['geo_secondary_role']` is set.
 If you want to run this database external to Omnibus, please follow the instructions below.
 
+If you are using a cloud-managed service for the tracking database, you may need
+to grant additional roles to your tracking database user (by default, this is
+`gitlab_geo`):
+
+- 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.
+
 The tracking database requires an [FDW](https://www.postgresql.org/docs/11/postgres-fdw.html)
 connection with the **secondary** replica database for improved performance.