doc: add note that vm module is not a security mechanism

the text added in this commit should warn users about
wrong idea that vm module can be secure to run unsafe scripts
in sandboxes

PR-URL: https://github.com/nodejs/node/pull/11557


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>