doc: revise os.md

Revise os.md, mostly making it more concise. In one case, process.md was
revised as well to be consistent with a revision in os.md.

PR-URL: https://github.com/nodejs/node/pull/30102


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>