Doc change: updates to Doze/Standby cases.

<p>The table below highlights the acceptable use cases for requesting or being on

   the Battery Optimizations exceptions whitelist. In general, your app should not be on the

   whitelist unless Doze and App Standby break the core function of the app, and there is a

   whitelist unless Doze or App Standby break the core function of the app or there is a

   technical reason why your app cannot use GCM high-priority messages.</p>

   <p>For more information, see <a href="#support_for_other_use_cases">Support for Other Use Cases

    <td rowspan="1">Instant messaging, chat, or calling app;

    enterprise VOIP apps</td>

    <td>No, can not use GCM because of technical dependency on another messaging

    service</td>

    service or Doze and App Standby break the core function of the app.</td>

    <td style="color:green">Acceptable</td>

    <td></td>

 </tr>