ocaml-sdls: enable MDX on eio/security_eio.mli, fix broken doc example
Run mdx on eio/security_eio.mli so the {[ ... ]} odoc block now
type-checks against the real Sdls.Security and Sdls_eio.Security_eio
APIs.
The example used unqualified `Security_eio.of_path`, `Security.*`,
and the bare constructor `Bad_mac`; the real path resolution is
`Sdls.Security.Bad_mac` (the constructor lives on
`Sdls.Security.auth_failure_reason`, not `Sdls.Mc`). Added a wrapper
`let run () = ...` so the file-backed security store does not
materialise on disk at mdx test time, and replaced the trailing `...`
with a `Printf.printf` of the dump length so the example shows what
the store yields.