Update doc for Future.whenComplete.

sdk/lib/async/future.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.async;
 
 /**
  * A [Future] represents a delayed computation. It is used to obtain a not-yet
  * available value, or error, sometime in the future.  Receivers of a
  * [Future] can register callbacks that handle the value or error once it is
@@ skipping 258 matching lines @@
    * The future returned by this call, `f`, will complete the same way
    * as this future unless an error occurs in the [action] call, or in
    * a [Future] returned by the [action] call. If the call to [action]
    * does not return a future, its return value is ignored.
    *
    * If the call to [action] throws, then `f` is completed with the
    * thrown error.
    *
    * If the call to [action] returns a [Future], `f2`, then completion of
    * `f` is delayed until `f2` completes. If `f2` completes with
-   * an error, that will be the result of `f` too.
+   * an error, that will be the result of `f` too. The value of `f2` is always
+   * ignored.
    *
    * This method is equivalent to:
    *
    *     Future<T> whenComplete(action()) {
    *       this.then((v) {
-   *                   action();
+   *                   var f2 = action();
+   *                   if (f2 is Future) return f2.then((_) => v);
    *                   return v
    *                 },
    *                 onError: (AsyncError e) {
-   *                   action();
+   *                   var f2 = action();
+   *                   if (f2 is Future) return f2.then((_) { throw e; });
    *                   throw e;
    *                 });
    *     }
    */
   Future<T> whenComplete(action());
 
   /**
    * Creates a [Stream] that sends [this]' completion value, data or error, to
    * its subscribers. The stream closes after the completion value.
    */
@@ skipping 43 matching lines @@
    * The argument [exception] should not be `null`.
    *
    * If [exception] is an [AsyncError], it is used directly as the error
    * message sent to the future's listeners, and [stackTrace] is ignored.
    *
    * Otherwise the [exception] and an optional [stackTrace] is combined into an
    * [AsyncError] and sent to this future's listeners.
    */
   void completeError(Object exception, [Object stackTrace]);
 }