Update user observer method description with instructions to check ex…

…ternalId

src/index.ts

@@ -266,7 +266,10 @@ export namespace OneSignal {
       }
     }
 
-    /** Add a callback that fires when the OneSignal user state changes. */
+    /** 
+     * Add a callback that fires when the OneSignal user state changes. 
+     * Important: When using the observer to retrieve the onesignalId, check the externalId as well to confirm the values are associated with the expected user.
+    */
     export function addEventListener(
       event: 'change',
       listener: (event: UserChangedState) => void,