doc: add blank line between comments

Upcoming remark-based formatting/linting will add a blank line between
comments in markdown. This is in preparation for that change.

PR-URL: https://github.com/nodejs/node/pull/40160


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>