Update KEF docs - adds supports_on (#11812)

* LSX is tested now

* add supports_on

* fix default

source/_integrations/kef.markdown

@@ -15,7 +15,7 @@ The `kef` platform allows you to control the [KEF LS50 Wireless](https://interna
 Supported devices:
 
 - KEF LS50 Wireless
-- KEF LSX (untested)
+- KEF LSX
 
 To add KEF Speakers to your installation, add the following to your `configuration.yaml` file:
 
@@ -66,6 +66,11 @@ standby_time:
   description: The speakers automatically turn to standby mode after either `20` or `60` minutes. Leave out for the speaker to never go into standby mode.
   required: false
   type: integer
+supports_on:
+  description: LS50 Wireless with a serial number below LS50W13074K24L/R2G do not support turning on the speakers over the network. Set this to false if you have an older model.
+  default: true
+  required: false
+  type: integer
 {% endconfiguration %}
 
 ## Advanced configuration example