ocaml-arp: enable MDX on lib/arp.mli, fix broken doc example
Run mdx on lib/arp.mli so the {[ ... ]} odoc block now type-checks.
The example used `entry.Arp.ip` for record-field access, but the
`Arp.` qualifier on a field only resolves through `open` or a type
annotation, not as a label prefix. Annotated `(entry : Arp.entry)`
and wrapped in `let print_entries () = ...` so reading the live
system ARP cache does not run at mdx test time.