doc: make the style of notes consistent
Make the style of "Note:" paragraphs consistent and document the guidelines in doc/STYLE_GUIDE.md
.
Fixes: https://github.com/nodejs/node/issues/13131
I picked the style that I find reasonable and which I had used once adding a note myself, but I don't have any strong opinions about the notes style except that it should be only one, so the formatting may be discussed in this PR.
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc