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: nodejs/node#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>

doc/api/vm.md

@@ -14,6 +14,9 @@ const vm = require('vm');
 JavaScript code can be compiled and run immediately or compiled, saved, and run
 later.
 
+*Note*: The vm module is not a security mechanism.
+**Do not use it to run untrusted code**.
+
 ## Class: vm.Script
 <!-- YAML
 added: v0.3.1