doc: revise style guide

Use italics for words-as-words within style guide. Other minor
improvements.

PR-URL: https://github.com/nodejs/node/pull/25778


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>