Add helpful message about how to use localizationsDelegates list (#46250)

dev/tools/localization/gen_l10n.dart

@@ -94,6 +94,10 @@ class @className {
   /// Returns a list of localizations delegates containing this delegate along with
   /// GlobalMaterialLocalizations.delegate, GlobalCupertinoLocalizations.delegate,
   /// and GlobalWidgetsLocalizations.delegate.
+  ///
+  /// Additional delegates can be added by appending to this list in
+  /// MaterialApp. This list does not have to be used at all if a custom list
+  /// of delegates is preferred or required.
   static const List<LocalizationsDelegate<dynamic>> localizationsDelegates = <LocalizationsDelegate<dynamic>>[
     delegate,
     GlobalMaterialLocalizations.delegate,