docs(messaging): update docs to use @pragma('vm:entry-point') annotation for messaging background handler

docs/cloud-messaging/receive.md

@@ -125,8 +125,10 @@ There are a few things to keep in mind about your background message handler:
 
 1. It must not be an anonymous function.
 2. It must be a top-level function (e.g. not a class method which requires initialization).
+3. It must be annotated with `@pragma('vm:entry-point')` right above the function declaration (otherwise it may be removed during tree shaking for release mode).
 
 ```dart
+@pragma('vm:entry-point')
 Future<void> _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
   // If you're going to use other Firebase services in the background, such as Firestore,
   // make sure you call `initializeApp` before using other Firebase services.

packages/firebase_messaging/firebase_messaging/example/lib/main.dart

@@ -42,7 +42,8 @@ import 'firebase_options.dart';
 ///
 /// To verify that your messages are being received, you ought to see a notification appearon your device/emulator via the flutter_local_notifications plugin.
 /// Define a top-level named handler which background/terminated messages will
-/// call.
+/// call. Be sure to annotate the handler with `@pragma('vm:entry-point')` above the function declaration.
+@pragma('vm:entry-point')
 Future<void> _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
   await Firebase.initializeApp(options: DefaultFirebaseOptions.currentPlatform);
   await setupFlutterNotifications();