add unsubscribe token

text/0000-custom-model-classes.md

@@ -91,13 +91,16 @@ subscribe and unsubscribe to notifications of underlying changes to the data. On
 any registered callback for the given identifier, and pass in the type of the notification, allowing the record the opportunity to repull the data if needed. There are no guarantees around the timing of the notification callback being called after `RecordData` informs the store of changes. We expect that in a modern Ember app with tracked properties, this wouldn't be the common path for tracking changes. 
 
 ```ts
+interface UnsubscribeToken {
+  unsubscribe(): void;
+}
+
 interface NotificationCallback {
   (identifier: Identifier, notificationType: 'attributes' | 'relationships' | 'errors'  | 'meta'  | 'unload'): void;
 }
 
 interface NotificationManager {
-  subscribe(identifier, NotificationCallback): void
-  unsubscribe(identifier, NotificationCallback): void
+  subscribe(identifier, NotificationCallback): UnsubscribeToken
  }
 ```