ocaml-monitor: enable MDX on lib/monitor.mli
Run mdx on lib/monitor.mli so the {[ ... ]} odoc block now type-checks.
`Monitor.Process.create` takes only `~env` (the example passed `~sw`
too), and `Monitor.Process.start` takes `~sw` (the example missed it).
Wrapped in `let run () =` so the spawned `echo` doesn't actually run
at mdx test time, with the parameter labels matching the real
backend signature.