docs(headlines-feed): add SourceFilterPage documentation

- Add detailed documentation to SourceFilterPage class
- Explain the purpose and functionality of the page
- Clarify what information is returned and persisted
- Remove unnecessary commented-out code

Author: fulleni

lib/headlines-feed/view/source_filter_page.dart

@@ -10,21 +10,24 @@ import 'package:flutter_news_app_mobile_client_full_source_code/l10n/l10n.dart';
 import 'package:ui_kit/ui_kit.dart';
 
 // Keys are defined in headlines_filter_page.dart and imported by router.dart
-// const String keySelectedSources = 'selectedSources';
-// const String keySelectedCountryIsoCodes = 'selectedCountryIsoCodes';
-// const String keySelectedSourceTypes = 'selectedSourceTypes';
-
+/// {@template source_filter_page}
+/// A page dedicated to selecting news sources for filtering headlines.
+///
+/// This page allows users to refine the displayed list of sources using
+/// country and source type capsules. However, these internal UI filter
+/// selections (country and source type) are *not* returned or persisted
+/// outside this page. Its sole purpose is to return the list of
+/// *explicitly checked* [Source] items.
+/// {@endtemplate}
 class SourceFilterPage extends StatelessWidget {
+  /// {@macro source_filter_page}
   const SourceFilterPage({
     super.key,
     this.initialSelectedSources = const [],
-    this.initialSelectedCountryIsoCodes = const {},
-    this.initialSelectedSourceTypes = const {},
   });
 
+  /// The list of sources that were initially selected on the previous page.
   final List<Source> initialSelectedSources;
-  final Set<String> initialSelectedCountryIsoCodes;
-  final Set<SourceType> initialSelectedSourceTypes;
 
   @override
   Widget build(BuildContext context) {