message: Add documentation to SendService.call().
diff: @@ -3594,7 +3594,15 @@ public class MslControl {
  this.requestService = new RequestService(ctx, msgCtx, in, out, Receive.NEVER, timeout);
  }
  - /* (non-Javadoc)
  + /**
  + * @return the established MSL channel or {@code null} if cancelled or
  + * interrupted.
  + * @throws MslException if there was an error creating or processing
  + * a message.
  + * @throws IOException if there was an error reading or writing a
  + * message.
  + * @throws TimeoutException if the thread timed out while trying to
  + * acquire the renewal lock.
  * @see java.util.concurrent.Callable#call()
  */
  @Override
  