docs(messaging, android): update inline documentation on behavior (#12948)

Author: russellwheatley

packages/firebase_messaging/firebase_messaging/lib/src/messaging.dart

@@ -87,6 +87,9 @@ class FirebaseMessaging extends FirebasePluginPlatform {
   /// This should be used to determine whether specific notification interaction
   /// should open the app with a specific purpose (e.g. opening a chat message,
   /// specific screen etc).
+  ///
+  /// on Android, if the message was received in the foreground, and the notification was
+  /// pressed whilst the app is in a background/terminated state, this will return `null`.
   Future<RemoteMessage?> getInitialMessage() {
     return _delegate.getInitialMessage();
   }

packages/firebase_messaging/firebase_messaging_platform_interface/lib/src/platform_interface/platform_interface_messaging.dart

@@ -141,6 +141,9 @@ abstract class FirebaseMessagingPlatform extends PlatformInterface {
   /// This should be used to determine whether specific notification interaction
   /// should open the app with a specific purpose (e.g. opening a chat message,
   /// specific screen etc).
+  ///
+  /// on Android, if the message was received in the foreground, and the notification was
+  /// pressed whilst the app is in a background/terminated state, this will return `null`.
   Future<RemoteMessage?> getInitialMessage() {
     throw UnimplementedError('getInitialMessage() is not implemented');
   }