ocaml-cookie: refine validation example and restore RFC 6265 @see
The previous validation block asserted via [Result.is_error]; rewrite
as an mdx toplevel session (printed types and values) so the example
reads like an OCaml top-level you can paste into utop.
Restore the [@see <RFC 6265>] tags removed in the prior commit by
moving them to the end of each module-level docstring, after the
[{2 ...}] section headings (odoc rejects headings inside [@see]).