add dartdoc

lib/src/tag/tagger.dart

@@ -350,6 +350,16 @@ class Tagger {
     }
   }
 
+  /// Tag if iOS/macOS plugin has migrated to Swift Package Manager (swiftpm).
+  ///
+  /// A plugin only needs to be swiftpm enabled if it has a native component, we
+  /// detect that if it has the `flutter.plugin.platforms.<os>.pluginClass` key
+  /// present in the pubspec.
+  ///
+  /// A plugin can share code and package-manager manifest between iOS and
+  /// macOS by specifying `flutter.plugin.platforms.<os>.sharedDarwinSource`.
+  ///
+  /// See https://docs.flutter.dev/packages-and-plugins/swift-package-manager/for-plugin-authors
   void swiftPackageManagerPluginTag(
       List<String> tags, List<Explanation> explanations) {
     if (!_usesFlutter) return;