Update demo/src/BillingService.php

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

Author: koriym

demo/src/BillingService.php

@@ -4,8 +4,16 @@
 
 namespace Demo;
 
+/**
+ * Service interface for handling billing operations
+ */
 interface BillingService
 {
+    /**
+     * Charges an order
+     *
+     * @return string The charge confirmation message
+     */
     #[WeekendBlock]
-    public function chargeOrder();
+    public function chargeOrder(): string;
 }