doc: add C++ style comments to the style guide

Adds instructions on how to format C++ comments to the C++ style guide,
as `cpplint.py` doesn't complain about C-style comments on the code.

PR-URL: https://github.com/nodejs/node/pull/17617


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>