doc: multiple updates to child_process.md

* use serial comma in child_process.md
* use American spelling of unsanitized in child_process.md
* hyphenate general-purpose in child_process.md
* remove errant commas in child_process.md

PR-URL: https://github.com/nodejs/node/pull/17990


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>