docs: add "boot completed" broadcast to list of DO NOTs for installing on external storage

<p class="caution"><strong>Caution:</strong> Although XML markup such as this will be ignored by

older platforms, you must be careful not to use programming APIs introduced in API Level 8

while your {@code minSdkVersion} is less than "8", unless you perform the work necessary to

provide backward compatiblity in your code. For information about building

provide backward compatibility in your code. For information about building

backward compatibility in your application code, see the <a

href="{@docRoot}resources/articles/backward-compatibility.html">Backward Compatibility</a>

article.</p>

    <dd>Your {@link android.app.admin.DeviceAdminReceiver} and all its admin capabilities will

be disabled, which can have unforeseeable consequences for the device functionality, which may

persist after external storage is remounted.</dd>

  <dt>Broadcast Receivers listening for "boot completed"</dt>

    <dd>The system delivers the {@link android.content.Intent#ACTION_BOOT_COMPLETED} broadcast

before the external storage is mounted to the device. If your application is installed on the

external storage, it can never receive this broadcast.</dd>

</dl>

<p>If your application uses any of the features listed above, you <strong>should not</strong> allow