carry the fork warning in the dune-project descriptions too
The README fork warning landed for these nine forks, but the established
standard also carries a WARNING: prefix in the package description (21 other
forks do). Add it to each main package's description crediting the upstream
(digestif, cohttp, checkseum, mirage-tcpip, mirage, kdf, osrelease,
ocaml-dockerfile, jsont) and regenerate the opam files. Secondary packages
(-eio, mrg, -js) are left as-is, matching the main-package-only convention
the other forks follow.