docs: add inline documentation where missing

src/Form/Service/ParticipantTransportationDiscountReplacementFieldHandler.php

@@ -10,6 +10,18 @@ use App\Form\Model\BookingDto;
 use App\Form\Service\Abstract\AbstractParticipantFieldHandler;
 use App\Service\ServiceAvailabilityCalculator;
 
+/**
+ * Handles automatic replacement of discounted transportation based on booking rules.
+ *
+ * This virtual field handler enforces the business rule that discounted self-organized
+ * transportation (PKW with negative price) is only available when both outbound AND
+ * inbound travel use PKW. When inbound changes to BUS, the outbound PKW is automatically
+ * replaced with the regular (non-discounted) PKW variant.
+ *
+ * Conversely, when inbound changes back to PKW, this handler restores the discounted
+ * variant if availability permits. Baby participants (age <= 2) are excluded from
+ * discounted transportation restoration.
+ */
 class ParticipantTransportationDiscountReplacementFieldHandler extends AbstractParticipantFieldHandler
 {
     public function __construct(