gh-159011: Update dummy thread lifecycle documentation

[nightcityblade]

Fixes #159011.

Summary

• clarify that dummy thread objects are removed from the active thread list after their alien threads terminate
• document that this behavior changed in Python 3.13

Tests

• uv run prek run --files Doc/library/threading.rst
• focused Sphinx dummy build with warnings treated as errors
• Python 3.14 runtime check confirming the dummy thread is removed from threading.enumerate() after termination

• Issue: Docs: threading says dummy thread objects are never deleted, outdated since 3.13 #159011

[python-cla-bot]

The following commit authors need to sign the Contributor License Agreement:

• nightcityblade@gmail.com

[BHUVANSH855]

CLA check is failing, sign CLA so that maintainers can begin with review process.

[nightcityblade]

CI follow-up: I rebased the one-file documentation change onto current main (5a22a62). The previous Docs failure was caused by the old branch snapshot predating the merged CPython custom changes-builder support; the threading.rst diff itself is unchanged.

Local validation on the rebased head:

• uv run prek run --files Doc/library/threading.rst — passed
• git diff --check — passed

The CLA signature still requires the account-holder action noted above.

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #35043706 | 📁 Comparing f02009f against main (5a22a62)

  🔍 Preview build  

1 file changed

± library/threading.html