Return all available travel documents (boarding passes, travel tickets, stay and car vouchers) for a confirmed booking. Documents are fetched on-demand from the provider and are never persisted. bookingId accepts the Mercury UUID or the raw upstream provider booking ID. A response item with availabilityStatus: 'not_yet_open' means the provider has not yet issued the document (e.g. check-in not open). carrier_retrieval_required supplies credentials for retrieving a ticket directly from the carrier. For format: 'url', payload identifies the provider asset; clients must request the item endpoint at tap time rather than open this value or append an access token. Cache-Control reflects a short TTL because boarding details can change up to gate time.