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: #30102
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: Trott