Document the fact that SendPort.send() dispatches messages immediately.

sdk/lib/isolate/isolate.dart

Index: sdk/lib/isolate/isolate.dart
diff --git a/sdk/lib/isolate/isolate.dart b/sdk/lib/isolate/isolate.dart
index 8c2b86e18b024b381f38da6f75348dddbf804484..d239ef2b322e65c3d1d412371a1063308785f8c8 100644
--- a/sdk/lib/isolate/isolate.dart
+++ b/sdk/lib/isolate/isolate.dart
@@ -485,6 +485,10 @@ abstract class SendPort implements Capability {
    * is also possible to send object instances (which would be copied in the
    * process). This is currently only supported by the dartvm.  For now, the
    * dart2js compiler only supports the restricted messages described above.
+   *
+   * The send happens immediately and doesn't block.  The corresponding receive
+   * port can receive the message as soon as its isolate's event loop is ready
+   * to deliver it, independently of what the sending isolate is doing.
    */
   void send(var message);