Update the itinerary content of a trip. Every field is left unchanged when omitted, except sections, which is replaced wholesale when given (there is no partial/deep merge of section content).

Note: This endpoint only works for trips using the advanced editor - see the editor field on Create a trip / Update trip; a basic-editor trip has no itinerary to update. A successful update also publishes the trip immediately, so the change reaches the participant-visible copy.