Mention matrix_coturn_turn_external_ip_address in examples/hosts

examples/hosts

@@ -1,4 +1,5 @@
 # We explicitly ask for your server's external IP address, because the same value is used for configuring Coturn.
+# If you'd rather use a local IP here, make sure to set up `matrix_coturn_turn_external_ip_address`.
 #
 # To connect using a non-root user (and elevate to root with sudo later),
 # replace `ansible_ssh_user=root` with something like this: `ansible_ssh_user=username become=true become_user=root`