Explain the option of manually defining your public IP in the documentation for Coturn

I was very surprised by the fact that a rather important configuration for coturn was "hidden" in the Hosts file, but not mentioned here. 

Therefore my suggestion is to explicitly mention it here, as I believe that is that natural for people to look.

docs/configuring-playbook-turn.md

@@ -15,6 +15,13 @@ matrix_coturn_enabled: false
 
 In that case, Synapse would not point to any Coturn servers and audio/video call functionality may fail.
 
+## Manually defining your public IP
+In the `hosts` file we explicitly ask for your server's external IP address when defining `ansible_host`, because the same value is used for configuring Coturn.
+If you'd rather use a local IP for `ansible_host`, make sure to set up `matrix_coturn_turn_external_ip_address` replacing `YOUR_PUBLIC_IP` with the pubic IP used by the server.
+
+```yaml
+matrix_coturn_turn_external_ip_address: "YOUR_PUBLIC_IP"
+```
 
 ## Using your own external Coturn server