DOC: Improved the docstring of pandas.plotting.radviz
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.plotting.radviz) ######################
################################################################################

Plot a multidimensional dataset in 2D.

Each `Series` in the `DataFrame` is represented as a evenly distributed
slice on a circle. Each data point is rendered in the circle according to
the value on each `Series`. Highly correlated `Series` in the `DataFrame`
are placed closer on the unit circle.

RadViz allow to project a N-dimensional data set into a 2D space where the
influence of each dimension can be interpreted as a balance between the
influence of all dimensions.

Parameters
----------
frame : :class:`DataFrame`
    Pandas object holding the data.
class_column : str
    Column name containing the name of the data point category.
ax : :class:`matplotlib.axes.Axes`, optional
    A plot instance to which to add the information.
color : list[str] or tuple[str], optional
    Assign a color to each category. Example: ['blue', 'green'].
colormap : str or :class:`matplotlib.colors.Colormap`, default None
    Colormap to select colors from. If string, load colormap with that
    name from matplotlib.
kwds : optional
    Options to pass to matplotlib scatter plotting method.

Returns
-------
axes: :class:`matplotlib.axes.Axes`

See Also
--------
:func:`pandas.plotting.andrews_curves` : Plot clustering visualization

Examples
--------
.. plot::
    :context: close-figs

    >>> df = pd.DataFrame({
    ...         'SepalLength': [6.5, 7.7, 5.1, 5.8, 7.6, 5.0, 5.4, 4.6,
    ...                            6.7, 4.6],
    ...         'SepalWidth': [3.0, 3.8, 3.8, 2.7, 3.0, 2.3, 3.0, 3.2,
    ...                            3.3, 3.6],
    ...         'PetalLength': [5.5, 6.7, 1.9, 5.1, 6.6, 3.3, 4.5, 1.4,
    ...                            5.7, 1.0],
    ...         'PetalWidth': [1.8, 2.2, 0.4, 1.9, 2.1, 1.0, 1.5, 0.2,
    ...                            2.1, 0.2],
    ...         'Category': ['virginica', 'virginica', 'setosa',
    ...                         'virginica', 'virginica', 'versicolor',
    ...                         'versicolor', 'setosa', 'virginica',
    ...                         'setosa']
    ...     })
    >>> rad_viz = pd.plotting.radviz(df, 'Category')

################################################################################
################################## Validation ##################################
################################################################################
```
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20169?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@fb556ed`). [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/20169/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20169?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20169   +/-   ##
=========================================
  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/20169?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_misc.py](https://codecov.io/gh/pandas-dev/pandas/pull/20169/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19taXNjLnB5) | `38.98% <ø> (ø)` | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20169?src=pr&el=footer). Last update [fb556ed...f326c06](https://codecov.io/gh/pandas-dev/pandas/pull/20169?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I just did the requested changes. Let me know if further modifications are needed. LGTM. Thanks!