doc: added comments to code

cherrypick_annotations/lib/src/singleton.dart

@@ -1,4 +1,28 @@
+/// An annotation to declare a class as a singleton.
+///
+/// This can be used to indicate that only one instance of the class
+/// should be created, which is often useful in dependency injection
+/// frameworks or service locators.
+///
+/// Example:
+/// ```dart
+/// @module()
+/// abstract class AppModule extends Module {
+///   @singleton()
+///   Dio dio() => Dio();
+/// }
+/// ```
+/// Сгенерирует код:
+/// ```dart
+/// final class $AppModule extends AppModule {
+///   @override
+///   void builder(Scope currentScope) {
+///     bind<Dio>().toProvide(() => dio()).singleton();
+///   }
+/// }
+/// ```
 // ignore: camel_case_types
-class singleton {
+final class singleton {
+  /// Creates a [singleton] annotation.
   const singleton();
 }