docs(messaging): add comments to sample code

Noticed by @mtalhabaig2
Supercedes #7103
Fixes #7073

Author: mikehardy

docs/messaging/usage/index.md

@@ -268,19 +268,23 @@ you can configure your `AppDelegate.m` file (see instructions below) to inject a
 import { AppRegistry } from 'react-native';
 import messaging from '@react-native-firebase/messaging';
 
+// Handle background messages using setBackgroundMessageHandler
 messaging().setBackgroundMessageHandler(async remoteMessage => {
   console.log('Message handled in the background!', remoteMessage);
 });
 
+// Check if app was launched in the background and conditionally render null if so
 function HeadlessCheck({ isHeadless }) {
   if (isHeadless) {
     // App has been launched in the background by iOS, ignore
     return null;
   }
 
+  // Render the app component on foreground launch
   return <App />;
 }
 
+// Your main application component defined here
 function App() {
   // Your application
 }