ocaml-gdocs: enable MDX on lib/gdocs.mli, fix broken doc example
Run mdx on lib/gdocs.mli so the {[ ... ]} odoc block now type-checks
against the real Gdocs.get / Markdown / Document API.
The example used `let token : Gauth.token = (* ... *) in`, where
`(* ... *)` is not OCaml syntax, and `Result.get_ok` to swallow
errors. Wrapped in `let run ~(token : Gauth.token) ~doc_id = ...` so
the user supplies the credentials, and pattern-matched on
[Gdocs.get]'s `(_, [`Msg of string]) result` so the example shows
both the success and failure paths.