Improve DartDocs for dart:core classes that may not be extended or implemented.

sdk/lib/core/null.dart

 // Copyright (c) 2013, 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.
 
 part of dart.core;
 
 /**
- * Class of the `null` value.
- *
- * The only instance of the [Null] class is the `null` object.
- *
- * It is an error for a class to extend or implement Null.
+ * The reserved word [:null:] denotes an object that is the sole instance of 
+ * this class.
+ * 
+ * It is a compile-time error for a class to attempt to extend or implement
+ * Null.
  */
 class Null {
   factory Null._uninstantiable() {
     throw new UnsupportedError('class Null cannot be instantiated');
   }
 
   /** Returns the string `"null"`. */
   String toString() => "null";
 }