tools: add mdn link for Iterator

PR-URL: https://github.com/nodejs/node/pull/10620


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michal Zasso <targos@protonmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>