Update shutil.rst to clarify copy performance on Windows

[NewUserHa]

Documentation

The original documention https://docs.python.org/3/library/shutil.html#shutil-platform-dependent-efficient-copy-operations about Platform-dependent efficient for Windows only mentioned that shutil.copyfile(), but which is a bad read and write loop still.

Actually the shutil.copy2() used the Windows API COPYFILE2, which is faster, and native call which means don't copy to memory on userspace, and support copy-on-write.
So this should be mentioned in documention to let people know rather than only knowing until read all the opened issues on github and source code of shutil.

Related info:
how shutil was implemented: #7160.
why only shutil.copy2() use Windows API COPYFILE2 while shutil.copyfile() not: #137074.
data that shows shutil.copy2() or Windows API COPYFILE2 can accelerate transfer speed: https://github.com/antirotor/speedcopy.
PR of new version copy2 implemented: #105055.

Linked PRs

• gh-142407: Update shutil.rst to clarify copy performance on Windows  #142408
• GH-142407: Doc: Clarify shutil.copyfile() does not use CopyFile2 on Windows #144354

[xiatianxuan]

Hello! This would be my first contribution to CPython.
I’d like to give this doc fix a try — is it okay if I work on it?
I plan to update the "Platform-dependent efficient copy operations" section in shutil.rst based on the info in this thread.

Thank you!