DOC: update the pandas.DataFrame.plot.box docstring
A bit late to the party, but I did it.

- [ ] PR title is "DOC: update the <your-function-or-method> docstring"
- [ ] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [ ] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
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- [ ] It has been proofread on language by another sprint participant

```
################################################################################
#################### Docstring (pandas.DataFrame.plot.box)  ####################
################################################################################

Make a box plot of the DataFrame columns.

A box plot is a method for graphically depicting
groups of numerical data through their quartiles.
The box extends from the Q1 to Q3 quartile values of the data,
with a line at the median (Q2). The whiskers extend from the edges
of box to show the range of the data. The position of the whiskers
is set by default to 1.5*IQR (IQR = Q3 - Q1) from the edges of the
box. Outlier points are those past the end of the whiskers.

For further details see Wikipedia's
entry for `boxplot <https://en.wikipedia.org/wiki/Box_plot>`_.

A consideration when using this chart is that the box and the whiskers
can overlap, which is very common when plotting small sets of data.

Parameters
----------
by : string or sequence
    Column in the DataFrame to group by.
**kwds : optional
    Additional keywords are documented in
    :meth:`pandas.DataFrame.plot`.

Returns
-------
axes : :class:`matplotlib.axes.Axes` or numpy.ndarray of them

See Also
--------
pandas.DataFrame.boxplot: Another method to draw a box plot.
pandas.Series.plot.box: Draw a box plot from a Series object.
matplotlib.pyplot.boxplot: Draw a box plot in matplotlib.

Examples
--------
Draw a box plot from a DataFrame with four columns of randomly
generated data.

.. plot::
    :context: close-figs

    >>> data = np.random.randn(25, 4)
    >>> df = pd.DataFrame(data, columns=list('ABCD'))
    >>> plot = df.plot.box()

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

Errors found:
	Errors in parameters section
		Parameters {'kwds'} not documented
		Unknown parameters {'**kwds'}
```

Due to issue #15079 I wasn't sure how to document the `by` argument. I will follow up to add it later.

Changed kwds description according to #20348.