DOC: improved the scatter method
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.DataFrame.plot.scatter)  ##################
################################################################################

A scatter plot with point size *s* and color *c*.

The coordinates of each point *x,y* are defined by two dataframe
columns and filled circles are used to represent each point.

Parameters
----------
x : column name or column position
    Horizontal and vertical coordinates of each point.
y : column name or column position
    Vertical coordinates of each point.
s : scalar or array_like, optional
    Size of each point.
c : label, column name or column position, optional
    Color of each point.
kwds : optional
    Keyword arguments to pass on to :py:meth:`pandas.DataFrame.plot`.

Returns
-------
axes : matplotlib.AxesSubplot or np.array of them

See Also
--------
matplotlib.pyplot.scatter : scatter plot using multiple input data
    formats.

Examples
--------

.. plot::
    :context: close-figs

    >>> from sklearn.datasets import load_iris
    >>> iris = load_iris()
    >>> df = pd.DataFrame(iris.data[:,:2],
    ...                   columns=iris.feature_names[:2])
    >>> df['species'] = load_iris().target
    >>> f = df.plot.scatter(x='sepal length (cm)',
    ...                     y='sepal width (cm)',
    ...                     c='species',
    ...                     colormap='viridis')

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

Docstring for "pandas.DataFrame.plot.scatter" 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.