Add clarification about SSO configuration settings

They weren't evident from the documentation itself.

Author: markus456

maxscale/reference/maxscale-rest-api/maxscale-rest-api.md

@@ -157,6 +157,10 @@ MaxScale implements support for OpenID Connect for authentication of REST-API
 clients. MaxScale implements both the Code Flow and the Implicit Flows from
 OpenID Connect.
 
+The OpenID Connect server is defined using the `admin_oidc_url` setting and the
+credentials used to access it are defined using `admin_oidc_client_id` and
+`admin_oidc_client_secret`.
+
 MaxScale uses the `admin_jwt_issuer` as the `client_id` in all OpenID Connect
 requests unless `admin_oidc_client_id` has been defined.