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

sdk/lib/core/bool.dart

 // Copyright (c) 2012, 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;
 
+/**
+ * The reserved words [:true:] and [:false:] denote objects that are the only
+ * instances of this class.
+ * 
+ * It is a compile-time error for a class to attempt to extend or implement
+ * bool.
+ */
 class bool {
   factory bool._uninstantiable() {
     throw new UnsupportedError(
         "class bool cannot be instantiated");
   }
+
+  /**
+   * Returns [:"true":] if the receiver is [:true:], or [:"false":] if the
+   * receiver is [:false:].
+   */
+  String toString();
 }