doc: revise introductory child_process text

This consolidates information about Windows environment variables and
has a few other smaller improvements (punctuation, present tense, etc.).

PR-URL: https://github.com/nodejs/node/pull/35344


Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>