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

dev/tools/localization/gen_l10n.dart

diff --git a/dev/tools/localization/gen_l10n.dart b/dev/tools/localization/gen_l10n.dart
index d5202e9..972e981 100644
--- a/dev/tools/localization/gen_l10n.dart
+++ b/dev/tools/localization/gen_l10n.dart
@@ -94,6 +94,10 @@
   /// 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,