Clarify behavior of async result in Future.delayed

Closes #29283

Change-Id: I3d375d7ba70394e46162ed7303aa24a2d7a63f66
Reviewed-on: https://dart-review.googlesource.com/72551
Commit-Queue: Nate Bosch <nbosch@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
diff --git a/sdk/lib/async/future.dart b/sdk/lib/async/future.dart
index 6b0385a..beb982e 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -289,7 +289,11 @@
    * Creates a future that runs its computation after a delay.
    *
    * The [computation] will be executed after the given [duration] has passed,
-   * and the future is completed with the result of the computation,
+   * and the future is completed with the result of the computation.
+   *
+   * If [computation] returns a future,
+   * the future returned by this constructor will complete with the value or
+   * error of that future.
    *
    * If the duration is 0 or less,
    * it completes no sooner than in the next event-loop iteration,