Clarify action of `core-service`

schemas/service.prs

@@ -40,6 +40,7 @@ State =
 ServiceDependency = <depends-on @depender any @dependee ServiceState>.
 
 ; Asserts that the service is a "core" service. If *not* specified for
-; a service X, the system acts as if `<depends-on X <service-state
-; <milestone core> running>>` were asserted.
+; a service X, where X is not `<milestone Y>` for some Y, the system
+; acts as if `<depends-on X <service-state <milestone core> running>>`
+; were asserted.
 ServiceMilestone = <core-service @serviceName any>.