Clarify the semantics of the @required annotation

pkg/meta/lib/meta.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Constants for use in metadata annotations such as `@protected`.
 ///
 /// See also `@deprecated` and `@override` in the `dart:core` library.
 ///
 /// Annotations provide semantic information that tools can use to provide a
 /// better user experience. For example, an IDE might not autocomplete the name
@@ skipping 49 matching lines @@
 ///   `this`.
 const _Protected protected = const _Protected();
 
 /// Used to annotate a named parameter `p` in a method or function `f`.
 /// Indicates that every invocation of `f` must include an argument
 /// corresponding to `p`, despite the fact that `p` would otherwise be an
 /// optional parameter.
 ///
 /// Tools, such as the analyzer, can provide feedback if
 /// * the annotation is associated with anything other than a named parameter,
-///   or
+/// * the annotation is associated with a named parameter in a method `m1` that
+///   overrides a method `m0` and `m0` defines a named parameter with the same
+///   name that does not have this annotation, or
 /// * an invocation of a method or function does not include an argument
 ///   corresponding to a named parameter that has this annotation.
 const _Required required = const _Required();
 
 class _Factory {
   const _Factory();
 }
 
 class _MustCallSuper {
   const _MustCallSuper();
 }
 
 class _Protected {
   const _Protected();
 }
 
 class _Required {
   const _Required();
 }