GH-142407: Doc: Clarify shutil.copyfile() does not use CopyFile2 on Windows

[xiatianxuan]

The current documentation for shutil implies that all copy functions benefit from fast kernel-space copying on Windows via the CopyFile2 API. However, shutil.copyfile() is an exception — it still uses a userspace read/write loop.

This PR clarifies that:

• shutil.copy(), copy2(), copytree(), and move() use CopyFile2 (when available)
• shutil.copyfile() does not, and remains a buffered userspace copy
• Provides practical guidance: prefer copy2() over copyfile() for better performance on Windows when metadata preservation is acceptable

The change has been verified by:

• Building the docs locally with make html (0 warnings)
• Confirming the rendered HTML displays the updated text correctly

Related to GH-142407.

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📚 Documentation preview 📚: https://cpython-previews--144354.org.readthedocs.build/

[python-cla-bot]

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