doc: unify format of iterables

Also, make signatures easier for copy-paste testing.

PR-URL: https://github.com/nodejs/node/pull/20036


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>