DOC: update the Index.get duplicates 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.Index.get_duplicates)  ###################
################################################################################

Extract duplicated index elements.

Returns a sorted list of index elements which appear more than once in
the index.

Returns
-------
array-like
    List of duplicated indexes.

See Also
--------
Index.duplicated : Return boolean array denoting duplicates.
Index.drop_duplicates : Return Index with duplicates removed.

Examples
--------

Works on different Index of types.

>>> pd.Index([1, 2, 2, 3, 3, 3, 4]).get_duplicates()
[2, 3]
>>> pd.Index([1., 2., 2., 3., 3., 3., 4.]).get_duplicates()
[2.0, 3.0]
>>> pd.Index(['a', 'b', 'b', 'c', 'c', 'c', 'd']).get_duplicates()
['b', 'c']
>>> dates = pd.to_datetime(['2018-01-01', '2018-01-02', '2018-01-03',
...                         '2018-01-03', '2018-01-04', '2018-01-04'],
...                        format='%Y-%m-%d')
>>> pd.Index(dates).get_duplicates()
DatetimeIndex(['2018-01-03', '2018-01-04'],
              dtype='datetime64[ns]', freq=None)

Sorts duplicated elements even when indexes are unordered.

>>> pd.Index([1, 2, 3, 2, 3, 4, 3]).get_duplicates()
[2, 3]

Return empty array-like structure when all elements are unique.

>>> pd.Index([1, 2, 3, 4]).get_duplicates()
[]
>>> dates = pd.to_datetime(['2018-01-01', '2018-01-02', '2018-01-03'],
...                        format='%Y-%m-%d')
>>> pd.Index(dates).get_duplicates()
DatetimeIndex([], dtype='datetime64[ns]', freq=None)

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

Docstring for "pandas.Index.get_duplicates" correct. :)

```

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.


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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20322?src=pr&el=h1) Report
> Merging [#20322](https://codecov.io/gh/pandas-dev/pandas/pull/20322?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/31afaf858604ab85665b54b92f40cef19d69a28d?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20322/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20322?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20322      +/-   ##
==========================================
- Coverage   91.72%    91.7%   -0.03%     
==========================================
  Files         150      150              
  Lines       49165    49165              
==========================================
- Hits        45099    45087      -12     
- Misses       4066     4078      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.09% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.86% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20322?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20322/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.66% <ø> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20322/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20322?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20322?src=pr&el=footer). Last update [31afaf8...ae6a2f4](https://codecov.io/gh/pandas-dev/pandas/pull/20322?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @csfarkas thanks for the quick update!