Be more explicit regarding invitation_only in the doc.

doc/Settings.md

@@ -74,7 +74,7 @@ You can chose between the following modes:
 ##### Invitation based registry
 
 Additionally to the setting in the admin panel, you can decide if registrations are only possible using an invitation code or not.
-To enable invitation based registration, you have to set the `invitation_only` setting in the [config/local.config.php](/help/Config) file.
+To enable invitation based registration, you have to set the `invitation_only` setting to `true` in the `system` section of the [config/local.config.php](/help/Config) file.
 If you want to use this method, the registration policy has to be set to either *open* or *requires approval*.
 
 #### Check Full Names