For Central API docs, show optional/required info

For example: https://github.com/getodk/central-backend/blob/0cd56948854ecc544a1b4af9efd88cf9ceee153d/docs/api.yaml#L11958 -- this is not surfaced in the rendered docs.

Start with docs/api.yaml around the linked line and inspect how the Central API schema is rendered in the Sphinx documentation. Confirm where optional and required information is lost, then verify the rendered API docs visibly show that information for the example endpoint.

Written by the indexing model from the issue text.