settings: Describe new variable expansion

Author: cmouse

data/updates.js

@@ -75,6 +75,7 @@ export const updates = {
 	variables_auth_variables_protocol: '2.4.0',
 	variables_login_variables_protocol: '2.4.0',
 	variables_owner_user_added: '2.4.0',
+	var_expand: '2.4.0',
 
 	/* Tags used in doveadm.js */