doc: mark readline promise implementation as experimental

Refs: https://github.com/nodejs/node/pull/37947#issuecomment-921538713

PR-URL: https://github.com/nodejs/node/pull/40211


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>