DOC: update the pd.Index.argsort docstring
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [X] PR title is "DOC: update the <your-function-or-method> docstring"
- [ ] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [X] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [X] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```
################################################################################
####################### Docstring (pandas.Index.argsort) #######################
################################################################################

Return the order of the indices.

We have an object with values and index. Using argsort method
returns a series of indices that would sort the index.

Parameters
----------
*args
    Passed to `numpy.ndarray.argsort`.
**kwargs
    Passed to `numpy.ndarray.argsort`.

Returns
-------
numpy.ndarray : Argsorted indices of the index

See also
--------
numpy.ndarray.argsort : Returns the indices that would sort this array.

Examples
--------
>>> s = pd.Series(data=[4,3,2,1],index=['c','b','a','d'])
>>> s.index.argsort()
array([2, 1, 0, 3], dtype=int64)

################################################################################
################################## Validation ##################################
################################################################################

Errors found:
        Errors in parameters section
                Parameters {'args', 'kwargs'} not documented
                Unknown parameters {'**kwargs', '*args'}
                Parameter "*args" has no type
                Parameter "**kwargs" has no type
```

If the validation script still gives errors, but you think there is a good reason
to deviate in this case (and there are certainly such cases), please state this
explicitly.

We did our best to explain *args and **kwargs

Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
