REGR: Fix signature of GroupBy.expanding

[rhshadrach]

Ref: #61352 (comment)

#61352 replaced *args and **kwargs in the signature of GroupBy.expanding. However I believe further arguments need to be added. We could also revert the PR instead.

[arthurlw]

Thanks for pointing this out! Just to confirm, which additional arguments do you think should be included in the signature/documentation?

[rhshadrach]

From the old docstrings:

Arguments are the same as :meth:DataFrame.rolling except that step cannot be specified.

I believe that's accurate, but we should confirm.

[arthurlw]

When I was writing the documentation, I referred to the Expanding class and the DataFrame.expanding signature. Both expect min_periods, axis, and method as parameters.

DataFrame.rolling accepts several additional arguments, such as window, center, and win_type, which don't apply to expanding windows.

Also the docstring above was added by me in #61274, though after reviewing this more carefully, I believe that was a mistake.

[rhshadrach]

Also the docstring above was added by me in #61274, though after reviewing this more carefully, I believe that was a mistake.

At my request!

I missed that Expanding is part of the MRO for ExpandingGroupby. The current signature looks correct to me. Closing.