Corrected kiosk mode config

Only got the documented steps to work if the [autologin] part was added, so I suppose it is missing from this documentation

doc/20-Advanced-Topics.md

@@ -425,6 +425,13 @@ If you want to show a specific Dashboard you can enforce this onto the kiosk use
 
 Replace Remote_Addr with the IP where the kiosk user is accessing the Web to restrict further usage from other IPs.
 
+using the `REMOTE_USER` variable also requires adding the following to the top of `/etc/icingaweb2/authentication.ini`:
+```
+[autologin]
+backend = external
+```
+
+
 ## Customizing the Landing Page <a id="landing-page"></a>
 
 The default landing page of `dashboard` can be customized using the environment variable `ICINGAWEB_LANDING_PAGE`.