docs(app-check, ios): note iOS SDK has no onTokenChanged so is no-op (#8765)

* docs(app-check, iOS): onTokenChanged

---------

Co-authored-by: Mike Hardy <[email protected]>

Author: MichaelVerdon

docs/app-check/usage/index.md

@@ -226,6 +226,22 @@ try {
 }
 ```
 
+### Listening for token changes
+
+On Android, you can subscribe to App Check token updates with `onTokenChanged`. On iOS this listener is not implemented yet – subscribing will no-op and log a warning. If you need to react to token changes on iOS, prefer polling `getToken` on demand or rely on automatic refresh.
+
+```javascript
+import { onTokenChanged, getToken } from '@react-native-firebase/app-check';
+
+// Android: receives updates. iOS: no-op (native API does not exist on the SDK).
+const unsubscribe = onTokenChanged(appCheckInstance, async ({ token }) => {
+  console.log('App Check token updated:', token);
+});
+
+// iOS-friendly approach: request a fresh token when needed
+const { token } = await appCheckInstance.getToken(true);
+```
+
 ## Automatic Data Collection
 
 App Check has an "tokenAutoRefreshEnabled" setting. This may cause App Check to attempt a remote App Check token fetch prior to user consent. In certain scenarios, like those that exist in GDPR-compliant apps running for the first time, this may be unwanted.