gh-136059: docs: pathlib: Mention that iterdir() is surprisingly not streaming

[nh2]

This undocumented gotcha can cause excessive memory usage when "iterating" over very large directories.

This is because iterdir() does

entries = list(scandir_it)

Fixing (if at all desired) will likel need least significant amounts of discussion and testing, so first document the behaviour.

See:

• #136059

• Issue: gh-136059

---

📚 Documentation preview 📚: https://cpython-previews--136060.org.readthedocs.build/

[python-cla-bot[bot]]

All commit authors signed the Contributor License Agreement.