DOC: update the pandas.DataFrame.plot.pie 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 pandas.DataFrame.plot.pie`
- [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 for "pandas.DataFrame.plot.pie" correct. :)
```
@Renton2017, could you please make a reference to a line number?  The keyword arguments must be mentioned in the docstring as `**kwargs`. Can you include a screenshot of the generated html? Hi Dimitri,

on line 2844 `**kwds` : optional 
meanwhile on line 2849 kwds  : optional 

They cannot be true both! Hello @dim1tri! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on March 16, 2018 at 21:46 Hours UTC @dim1tri please ping me if you have issues fixing the merge conflicts. @dim1tri is there something we can do to help you with this PR? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20133?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@3561580`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20133/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20133?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20133   +/-   ##
=========================================
  Coverage          ?   91.72%           
=========================================
  Files             ?      150           
  Lines             ?    49152           
  Branches          ?        0           
=========================================
  Hits              ?    45086           
  Misses            ?     4066           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.11% <ø> (?)` | |
| #single | `41.84% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20133?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/20133/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.23% <ø> (ø)` | |

------

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 Updated

Fixed the aspect ratio on the plots, added a `subplots=True` example, and used the `index` instead of labels.

![fireshot capture 006 - pandas dataframe plot pie pandas 0 _ - file____users_taugspurger_sandbox_](https://user-images.githubusercontent.com/1312546/37546150-8d5c5cb8-2939-11e8-8e19-e0e3b2c0c0d4.png)
 @dim1tri Thanks! I have wanted to ask for a while, but never sure where.. Is this documentation page a joke? Is it intentionally trolling? It is, right?