gh-82419: improve doc for ENABLE_USER_SITE

[damngamerz]

• improve doc according to the issue.

Not sure if did it correctly. My suggestion would be to just deprecate the whole ENABLE_USER_SITE configuration.

• Issue: site.py reporting user site info even if ENABLE_USER_SITE=False #82419

---

📚 Documentation preview 📚: https://cpython-previews--121684.org.readthedocs.build/

[ghost]

All commit authors signed the Contributor License Agreement.

[encukou]

AFAIK, changing site.py file won't work (unless you recompile Python), but administrators should still be able to set site.ENABLE_USER_SITE in a sitecustomize module.

[encukou]

Thanks for the contribution, but as it is, the addition is misleading.
I'll close this PR. Feel free to discuss on the issue, and open a new PR when ready.