Make controller documentation more explicit about sync.
Make controller documentation more explicit about sync.

R=lrn@google.com

Review-Url: https://codereview.chromium.org/2831133002 .

dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk