I have transitioned to using GraphQL APIs exclusively, including for the new ATF backend. REST APIs tend to be fairly simple to document, plus they have generators to help. GraphQL APIs are more verbose and more difficult to document, but they also have a few options for document generators. The one I have recently began using, actually for the ATF backend, is one named SpectaQL (pronounced Spectacle). It wasn’t the first generator I discovered, but it is the best [that I’ve found]. It involves minimum configuration and it runs well as installed globally or as a dev dependency.
I don’t have a public facing URL to demonstrate, yet, but even a GraphQL schema with ~65 object types generated almost flawlessly. Then it accepts metadata to fill in the gaps. If you are looking for a GraphQL document generator, I definitely recommend it.