Add comment to site_settings.yml about type: list

config/site_settings.yml

@@ -10,6 +10,7 @@
 #           Set to the class name that defines the set.
 # type: email    - Must be a valid email address.
 # type: username - Must match the username of an existing user.
+# type: list     - A list of values, chosen from a set of valid values defined in the choices option.
 required:
   title:
     client: true