docs(firebase_auth): explicit mention that refreshToken is empty string on native platforms on the Userinstance (#9183)

Author: russellwheatley

packages/firebase_auth/firebase_auth/lib/src/user.dart

@@ -73,8 +73,8 @@ class User {
 
   /// Returns a JWT refresh token for the user.
   ///
-  /// This property maybe `null` or empty if the underlying platform does not
-  /// support providing refresh tokens.
+  /// This property will be an empty string for native platforms (android, iOS & macOS) as they do not
+  /// support refresh tokens.
   String? get refreshToken {
     return _delegate.refreshToken;
   }

packages/firebase_auth/firebase_auth_platform_interface/lib/src/platform_interface/platform_interface_user.dart

@@ -89,8 +89,8 @@ abstract class UserPlatform extends PlatformInterface {
 
   /// Returns a JWT refresh token for the user.
   ///
-  /// This property maybe `null` or empty if the underlying platform does not
-  /// support providing refresh tokens.
+  /// This property will be an empty string for native platforms (android, iOS & macOS) as they do not
+  /// support refresh tokens.
   String? get refreshToken {
     return _user['refreshToken'];
   }