doc: consolidate -j information for make
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