Adds information for OwnTracks friends via HTTP (home-assistant#10797)

* Adds config information for OwnTracks friends

* Owntracks friends is disabled by default

* Updates the OwnTracks documentation to reflect latest code changes in the PR

source/_integrations/owntracks.markdown

@@ -10,7 +10,7 @@ ha_config_flow: true
 
 [OwnTracks](https://owntracks.org/) is a free and open source application for iOS and Android that allow you to track your location and send it directly to Home Assistant. It can be set up via the integrations panel in the configuration screen.
 
-By default the integration will listen for incoming messages from OwnTracks via HTTP. It will also listen for MQTT messages if Home Assistant is configured to use MQTT.
+By default the integration will listen for incoming messages from OwnTracks via HTTP. It will also listen for MQTT messages if Home Assistant is configured to use MQTT. When a location is submitted via HTTP, Home Assistant will return all [Persons](/integrations/person/)' last known locations and they will be displayed within the OwnTracks app.
 
 <div class='videoWrapper'>
 <iframe width="560" height="315" src="https://www.youtube.com/embed/UieAQ8sC6GY" frameborder="0" allowfullscreen></iframe>