ocaml-cache: enable MDX on lib/cache.mli, fix broken doc example
Run mdx on lib/cache.mli so the {[ ... ]} odoc block now type-checks.
The example referenced `env` out of nowhere when calling
`Eio.Stdenv.clock env`. Wrapped in `let run () = Eio_main.run @@ ...`
so the env is bound, and replaced the trailing `(* Some "answer" *)`
prose comment with `assert (MyCache.get cache 42 = Some "answer")`
so the documented round-trip is verified at mdx test time.