Update the TrainSchedule model design documentation for arrival on stop signal

[woshilapin]

Description and goal

The TrainSchedule model must be updated to take into account the arrival on stop signal.

Hint: This will probably take place in the schedule section of https://osrd.fr/en/docs/reference/design-docs/timetable/#modifiable-fields as a boolean.

Acceptance criteria:

• [x] Design documentation has been updated