Add documentation of List constructor.

lib/core/list.dart

Index: lib/core/list.dart
diff --git a/lib/core/list.dart b/lib/core/list.dart
index 6a34cbc28dac4d0f1f23265ac4800077e1368f41..e44e743974677c650ce08f4dc1621155ec264c06 100644
--- a/lib/core/list.dart
+++ b/lib/core/list.dart
@@ -1,4 +1,4 @@
-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 
@@ -10,6 +10,12 @@ interface List<E> extends Collection<E> default ListImplementation<E> {
 
   /**
    * Creates a list of the given [length].
+   *
+   * If no [length] argument is supplied an extendable list of
+   * length 0 is created.
+   *
+   * If a [length] argument is supplied, a fixed size list of that
+   * length is created.
    */
   List([int length]);