[v14.x backport] 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
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