You still need to write a spec with protobufs. It's just that the spec is much more succinct and easy to read. And you can generate docs from it. E.g.:

https://cloud.google.com/appengine/docs/admin-api/reference/...

Do an experiment, take a Protobuf spec for some Google API and convert it into OpenAPI.

We have used the gem: https://github.com/a-chacon/oas_rails, and we didn't need to write the specs, just comment the endpoints