doc: fix code nits in common/README

1. Sync comments and code
2. Fix typos

Backport-PR-URL: https://github.com/nodejs/node/pull/20456
PR-URL: https://github.com/nodejs/node/pull/17971


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>