DOC/DEPR: ensure that @deprecated functions have correct docstring
Follow-up on https://github.com/pandas-dev/pandas/pull/16955, the Series.argmin/argmax now have a docstring.

Currently:
```
In [1]: pd.Series.argmax?
Signature: pd.Series.argmax(*args, **kwargs)
Docstring: <no docstring>
File:      ~/scipy/pandas/pandas/util/_decorators.py
Type:      function
```

With this PR: 
```
In [1]: pd.Series.argmax?
Signature: pd.Series.argmax(self, axis=None, skipna=True, *args, **kwargs)
Docstring:
'argmax' is deprecated, use 'idxmax' instead. The behavior of 'argmax'
will be corrected to return the positional maximum in the future. Use
'series.values.argmax' to get the position of the maximum now.

Index *label* of the first occurrence of maximum of values.

Parameters
----------
skipna : boolean, default True
    Exclude NA/null values

Returns
-------
idxmax : Index of maximum of values

Notes
-----
This method is the Series version of ``ndarray.argmax``. This method
returns the label of the maximum, while ``ndarray.argmax`` returns
the position. To get the position, use ``series.values.argmax()``.

See Also
--------
DataFrame.idxmax
numpy.ndarray.argmax
File:      ~/scipy/pandas/pandas/core/series.py
Type:      function
```