src: explain implementation of vm module

The vm module uses interceptors on the object template. This is not
straight forward and a comment in the source will help the next
person working on this.

PR-URL: https://github.com/nodejs/node/pull/16962


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>