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
```
Hello @jorisvandenbossche! Thanks for updating the PR.



 - In the file [`pandas/util/_decorators.py`](https://github.com/pandas-dev/pandas/blob/ab2ed84ea0dbf064d4e0f01bf4e5eb145d63fc2a/pandas/util/_decorators.py), following are the PEP8 issues :

> [Line 41:28](https://github.com/pandas-dev/pandas/blob/ab2ed84ea0dbf064d4e0f01bf4e5eb145d63fc2a/pandas/util/_decorators.py#L41): [W503](https://duckduckgo.com/?q=pep8%20W503) line break before binary operator



##### Comment last updated on November 13, 2017 at 23:33 Hours UTC This needs https://github.com/pandas-dev/pandas/pull/18256 to be merged first # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18215?src=pr&el=h1) Report
> Merging [#18215](https://codecov.io/gh/pandas-dev/pandas/pull/18215?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/69472f9ae9c645a0a5932c7c65b6efbdf66fbb5f?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18215/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18215?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18215      +/-   ##
==========================================
- Coverage    91.4%   91.38%   -0.02%     
==========================================
  Files         164      164              
  Lines       49878    49880       +2     
==========================================
- Hits        45590    45583       -7     
- Misses       4288     4297       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.19% <100%> (ø)` | :arrow_up: |
| #single | `39.42% <100%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18215?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/18215?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `95% <ø> (ø)` | :arrow_up: |
| [pandas/util/\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/18215?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RlY29yYXRvcnMucHk=) | `81.03% <100%> (+0.33%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18215?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18215?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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