docs: add inline documentation where missing

src/Validator/Constraints/PromoVoucherValidator.php

@@ -11,6 +11,13 @@ use Symfony\Component\Validator\Constraint;
 use Symfony\Component\Validator\ConstraintValidator;
 use Symfony\Component\Validator\Exception\UnexpectedTypeException;
 
+/**
+ * Validates promotional voucher codes against the BPN API.
+ *
+ * Verifies that promo codes are valid for the current travel, checks minimum price
+ * thresholds, and validates code availability. Uses the participant's calculated
+ * total price (including room and all selected services) for threshold comparison.
+ */
 class PromoVoucherValidator extends ConstraintValidator
 {
     public function __construct(