doc: make the style of notes consistent

Make the style of "Note:" paragraphs consistent and document the
guidelines in `doc/STYLE_GUIDE.md`.

PR-URL: https://github.com/nodejs/node/pull/13133
Fixes: https://github.com/nodejs/node/issues/13131


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>