gh-156950: Clarify defaultdict documentation

[GruffElixir]

Summary

• Clarify how defaultdict handles missing keys when default_factory is set.
• Document that default_factory should be callable or None, and that other values raise TypeError.

Fixes gh-156950

Testing

• sphinx-lint --enable default-role Doc/library/collections.rst
• git diff --check
• Runtime smoke checks for callable, None, and invalid factories.

• Issue: Defaultdict docs could be clearer #156950

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[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #34417961 | 📁 Comparing f0460ce against main (83dbe6a)

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1 file changed

± library/collections.html