DOC: update the pandas.Series.sort_index 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"
- [x] 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.Series.sort_index) #####################
################################################################################

Sort Series by index labels.

Returns a new Series sorted by label if `inplace` argument is `False`,
otherwise updates the original series and returns `null`.

Parameters
----------
axis : int, default 0
    Axis to direct sorting. This can only be 0 for Series.
level : int, optional
    If not None, sort on values in specified index level(s).
ascending : bool, default true
    Sort ascending vs. descending.
inplace : bool, default False
    If True, perform operation in-place.
kind : {'quicksort', 'mergesort', 'heapsort'}, default 'quicksort'
    Choice of sorting algorithm. See also :func:`numpy.sort` for more
    information.  'mergesort' is the only stable algorithm. For
    DataFrames, this option is only applied when sorting on a single
    column or label.
na_position : {'first', 'last'}, default 'last'
    If 'first' puts NaNs at the beginning, 'last' puts NaNs at the end.
    Not implemented for MultiIndex.
sort_remaining : bool, default True
    If true and sorting by level and index is multilevel, sort by other
    levels too (in order) after sorting by specified level.

Returns
-------
pandas.Series
    The original Series sorted by the labels

See Also
--------
DataFrame.sort_index: Sort DataFrame by the index
DataFrame.sort_values: Sort DataFrame by the value
Series.sort_values : Sort Series by the value

Examples
--------
>>> s = pd.Series(['a', 'b', 'c', 'd'], index=[3, 2, 1, 4])
>>> s.sort_index()
1    c
2    b
3    a
4    d
dtype: object

Sort Descending

>>> s.sort_index(ascending=False)
4    d
3    a
2    b
1    c
dtype: object

Sort Inplace

>>> s.sort_index(inplace=True)
>>> s
1    c
2    b
3    a
4    d
dtype: object

By default NaNs are put at the end, but use `na_position` to place
them at the beginning

>>> s = pd.Series(['a', 'b', 'c', 'd'], index=[3, 2, 1, np.nan])
>>> s.sort_index(na_position='first')
NaN     d
 1.0    c
 2.0    b
 3.0    a
dtype: object

Specify index level to sort

>>> arrays = [np.array(['qux', 'qux', 'foo', 'foo',
...                     'baz', 'baz', 'bar', 'bar']),
...           np.array(['two', 'one', 'two', 'one',
...                     'two', 'one', 'two', 'one'])]
>>> s = pd.Series([1, 2, 3, 4, 5, 6, 7, 8], index=arrays)
>>> s.sort_index(level=1)
bar  one    8
baz  one    6
foo  one    4
qux  one    2
bar  two    7
baz  two    5
foo  two    3
qux  two    1
dtype: int64

Does not sort by remaining levels when sorting by levels

>>> s.sort_index(level=1, sort_remaining=False)
qux  one    2
foo  one    4
baz  one    6
bar  one    8
qux  two    1
foo  two    3
baz  two    5
bar  two    7
dtype: int64

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

Docstring for "pandas.Series.sort_index" correct. :)
```