allow specifiying more extra owner roles

docs/reference/operator_parameters.md

@@ -177,12 +177,14 @@ under the `users` key.
   Postgres username used for replication between instances. The default is
   `standby`.
 
-* **cron_admin_username**
-  Specifies the role that owns rights to set up cron jobs with `pg_cron`
-  extension inside the `postgres` database. The must be pre-configured in the
-  docker image. In Spilo this role is called `cron_admin`. This role will
-  become a member of all database owners so that they can set up cron jobs
-  e.g. as part of a migration script. Default is `empty`.
+* **additional_owner_roles**
+  Specifies database roles that will become members of all database owners.
+  Then owners can use `SET ROLE` to obtain privileges of these roles to e.g.
+  create/update functionality from extensions as part of a migration script.
+  Note, that roles listed here should be preconfigured in the docker image
+  and already exist in the database cluster on startup. One such role can be
+  `cron_admin` which is provided by the Spilo docker image to set up cron
+  jobs inside the `postgres` database. Default is `empty`.
 
 * **enable_password_rotation**
   For all `LOGIN` roles that are not database owners the operator can rotate
@@ -755,7 +757,7 @@ key.
 
 * **protected_role_names**
   List of roles that cannot be overwritten by an application, team or
-  infrastructure role. The default is `admin`.
+  infrastructure role. The default list is `admin` and `cron_admin`.
 
 * **postgres_superuser_teams**
   List of teams which members need the superuser role in each PG database