DOC: Clarify DataFrame.dropna lib.no_default kwarg behavior
- [x] closes #55848

Changes
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To
<img width="917" alt="Screenshot 2023-11-11 at 9 42 12 PM" src="https://github.com/pandas-dev/pandas/assets/25352646/e53c7b6a-e9a6-48e7-8a3a-dfb518d39990">

by adding one-line function declaration to doc-string per sphinx default autodoc_docstring_signature behavior.
@kamilkrukowski I don't think the changes in this PR make sense. Not sure if the solution you want to implement is as easy as you think. I agree with your points in the issue, but as Thomas said, we want to keep the current behavior, even if unfortunately exposes some confusing internal details to the users.

If you want to change the header rendered by Sphinx you will have to do it with a Sphinx plugin. Adding or changing things in the docstring won't change the signature. Only changing the Python code signature, which we don't want.

I'll close this issue, happy to follow up if I misunderstood what you are doing, or you want to discuss further. But I think what you want to achieve is too complex to be worth. So, unfortunately users will have to live with the nodefault argument in the documentation.