ocaml-http: enable MDX on lib/expect_continue.mli
Run mdx on lib/expect_continue.mli so the {[ ... ]} odoc block now
type-checks. The example bound `let session = Requests.v ~sw ... env`
three times (each shadowing the last) with `~sw` and `env` free.
Wrapped each variant in its own `let run_X () = Eio_main.run @@ ...`
so the toplevel doesn't actually open three sessions, and each
function shows one of the configuration choices on its own.