doc: fix various nits

* Replace 2 hyphens (--) by spaced m-dashes (—) as per STYLE_GUIDE.md.
* Space infix operators.
* Unify quotes in inline code spans (use only single quotes).
* Unify `* Returns:` (eliminate deviations).
* Dedupe spaces.

Backport-PR-URL: https://github.com/nodejs/node/pull/19761
PR-URL: https://github.com/nodejs/node/pull/19743


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