Passing extra keyword arguments to `curvefit` throws an exception.

What happened?

Just like the title says, passing an extra keyword argument corresponding to scipy's curve_fit throws an exception. The documentation has a parameter section that says:

*kwargs (optional) – Additional keyword arguments to passed to scipy curve_fit

So if one specifies a method="trf" keyword argument to the .curvefit method, you get an error: TypeError: curvefit() got an unexpected keyword argument 'method'

The only way it works as expected is if I pass the keyword arguments as dictionary elements via a kwargs argument like so: kwargs={"method": "trf"}. This behaviour contradicts what is mentioned in the docstring.

What did you expect to happen?

No error thrown

Minimal Complete Verifiable Example

import pandas as pd
import xarray as xr
import numpy as np

da = xr.DataArray(

    np.random.rand(4, 3),

    [

        ("time", pd.date_range("2000-01-01", periods=4)),

        ("space", ["IA", "IL", "IN"]),

    ],

)
da.curvefit(coords=["time"], func=lambda x, params: x, method="trf")

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

TypeError: curvefit() got an unexpected keyword argument 'method'

Anything else we need to know?

No response

Environment

</details>

Start at the DataArray.curvefit entry point and compare its documented *kwargs behavior with the scipy curve_fit call described in the issue. Verify the existing kwargs dictionary form and the direct method="trf" form, then run the curvefit tests or a minimal example to confirm that the documented call completes without an unexpected-keyword error.