doc: consolidate -j information for make

[Trott]

Our docs contain a mixture of make -j4, make -j8, and make without
a -j option at all.

Standardize on not specifying a -j option, but include information in
BUILDING.md (where people are likely to be looking the first time they
build Node.js) about setting -j, including the possibility of using
MAKEFLAGS.

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

build doc meta

[gibfahn]

@Trott I'm pretty sure we already had this discussion and standardized in #9961. Not sure why the -j4 wasn't added in some places.

Basically the consensus in that PR was that we should standardize on -j4, because people who don't know about the -j option are likely to have 4 virtual cores. If you are running on something with less then it shouldn't slow you down that much.

I think it's better to have -j4 and a note about changing it than the other way around, most new contributors (in my experience) just copy and paste the commands, without reading the paragraphs in between.

EDIT: See also #8286

[Trott]

The -j8 in COLLABORATOR_GUIDE is now -j4 so good enough for me. Closing.