Update to reflect new Modes trait. (#7683)

* Update to reflect new Modes trait.

Update docs to reflect new Modes trait added to media_players.

* missed option add.

source/_components/google_assistant.markdown

@@ -185,10 +185,19 @@ Currently, the following domains are available to be used with Google Assistant,
 - light (on/off/brightness/rgb color/color temp)
 - lock (lock/unlock (to allow assistant to unlock, set the `allow_unlock` key in configuration))
 - cover (on/off/set position (via set brightness))
-- media_player (on/off/set volume (via set brightness))
+- media_player (on/off/set volume (via set brightness)/source (via set input source))
 - climate (temperature setting)
 - vacuum (dock/start/stop/pause)
 
+### {% linkable_title Media Player Sources %}
+
+Media Player sources are sent via the Modes trait in Google Assistant.  
+There is currently a limitation with this feature that requires a hard-coded set of settings. Because of this, the only sources that will be usable by this feature are listed here:  
+https://developers.google.com/actions/reference/smarthome/traits/modes
+
+#### Example Command:
+"Hey Google, change input source to TV on Living Room Receiver"
+
 ### {% linkable_title Troubleshooting the request_sync service %}
 
 The request_sync service requires that the initial sync from Google includes the agent_user_id. If not, the service will log an error that reads something like "Request contains an invalid argument". If this happens, then [unlink the account](https://support.google.com/googlehome/answer/7506443) from Home Control and relink.