Doc: clarify os.rename doesn't atomically replace dst contents (gh-143909)

[Yashp002]

Docs claim atomic overwrite when dst exists. Wrong—Unix leaves old contents readable briefly (new dst appears first, old unlinked after). Matches POSIX but needs explicit warning.

Replaces vague "atomic" block with namespace vs content distinction. No behavior change.

Fixes #143909. CC @pablogsal @a12k1

• Issue: os.rename documentation about atomic replace misleading #143909

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📚 Documentation preview 📚: https://cpython-previews--143937.org.readthedocs.build/

[picnixz]

Please. Stop creating PRs before a triager or a core dev triaged the issue. The change here is too verbose. And stop using LLMs.