docs: Noted how apps behave differntly on devices without buttons.

<p>If the user does not interact with your app for a period of time while it is displayed, or if

the user covers the screen with their palm, the system switches the activity to ambient mode.

After the app switches to ambient mode, update the activity UI to a more basic layout to reduce

power consumption. You should use a black background with minimal white graphics and text. To

ease a user into the transition from interactive to ambient mode, try to maintain similar placement

of items on the screen. For more information on presenting content on an ambient screen, see the

power consumption. You should use a black background with minimal white graphics and text.

 To ease a user into the transition from interactive to ambient mode, try to maintain similar

 placement of items on the screen. For more information on presenting content on an ambient screen,

 see the

<a href="{@docRoot}design/wear/watchfaces.html#DisplayModes">Watch Faces for Android Wear</a>

design guide.

design guide.</p>

<p> Note that when your app runs on a device without a hardware button, palming the screen does not

switch an app into ambient mode. Rather, it causes the app to exit and the home screen to appear.

This behavior is intended to ensure that users can exit apps gracefully. However, these devices still

 go to ambient mode when the screen times out.

</p>

<p class="note"><b>Note:</b> In ambient mode, disable any interactive elements on the