docs: add inline documentation where missing

src/Validator/Constraints/PurchaseVoucherValidator.php

@@ -10,6 +10,12 @@ use Symfony\Component\Validator\Constraint;
 use Symfony\Component\Validator\ConstraintValidator;
 use Symfony\Component\Validator\Exception\UnexpectedTypeException;
 
+/**
+ * Validates purchase voucher codes against the BPN API.
+ *
+ * Verifies that voucher codes exist and have remaining balance. Unlike promo
+ * vouchers, purchase vouchers do not require travel or price context validation.
+ */
 class PurchaseVoucherValidator extends ConstraintValidator
 {
     public function __construct(