Docs: update docs of copy

[shihai1991]

Updating reason: I meet many python developers who use copy module, but they don't know the copy module don't copy or deepcopy the immutable object. So I update this doc simply to make sure the user undertand this point clearly.

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📚 Documentation preview 📚: https://cpython-previews--105547.org.readthedocs.build/

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[serhiy-storchaka]

This is incorrect place for "immutable object". "Does not copy" means that it raises an exception for such types. For immutable objects the behavior is similar to functions and classes, except that it is natural and does not need rational. So, if add clarification for immutable objects, I would insert a separate sentence between the cases "does not copy" and "copy" in quotes. The latter would need to be modified to avoid repeating.

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