docs: Update MessageAPI notes to mention the unreliable transmission nature.

  <dd>The <a href="{@docRoot}reference/com/google/android/gms/wearable/MessageApi.html"><code>MessageApi</code></a> class

  can send messages and is good for remote procedure calls (RPC), such as controlling a handheld's

  media player from the wearable or starting an intent on the wearable from the handheld.

  The system always delivers the message when the handheld and wearable are connected and delivers

  an error when the devices are disconnected. Messages are great for one-way requests or for a

  request/response communication model.</dd>

  Messages are also great for one-way requests or for a request/response communication model.

  If the handheld and wearable are connected, the system queues the message for delivery and

  returns a successful result code. If the devices are not connected, an error is returned. A

  successful result code does not indicate that the message was delivered successfully as the

  devices may disconnect after receiving the result code.

</p></dd>

  <dt><b>Asset</b></dt>

  <dd><a href="{@docRoot}reference/com/google/android/gms/wearable/Asset.html"><code>Asset</code></a> objects are for