DOC: update the pandas.DataFrame.hist docstring
- [x] PR title is "DOC: update the pandas.DataFrame.hist docstring"
- [x] The validation script passes: `scripts/validate_docstrings.py pandas.DataFrame.hist`
- [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 pandas.DataFrame.hist`
- [x] It has been proofread on language by another sprint participant

```
################################################################################
###################### Docstring (pandas.DataFrame.hist)  ######################
################################################################################

Draw histogram of the DataFrame's series using matplotlib / pylab.

A histogram is a representation of the distribution of numerical data.
This function wraps the matplotlib histogram function for each serie in
the DataFrame. It returns an array with a plot for each histogram.

Parameters
----------
data : DataFrame
    The pandas object holding the data.
column : string or sequence
    If passed, will be used to limit data to a subset of columns.
by : object, optional
    If passed, then used to form histograms for separate groups.
grid : boolean, default True
    Whether to show axis grid lines.
xlabelsize : int, default None
    If specified changes the x-axis label size.
xrot : float, default None
    Rotation of x axis labels.
ylabelsize : int, default None
    If specified changes the y-axis label size.
yrot : float, default None
    Rotation of y axis labels.
ax : Matplotlib axes object, default None
    The histogram axes.
sharex : boolean, default True if ax is None else False
    In case subplots=True, share x axis and set some x axis labels to
    invisible; defaults to True if ax is None otherwise False if an ax
    is passed in; Be aware, that passing in both an ax and sharex=True
    will alter all x axis labels for all subplots in a figure!.
sharey : boolean, default False
    In case subplots=True, share y axis and set some y axis labels to
    invisible.
figsize : tuple
    The size of the figure to create in inches by default.
layout : tuple, optional
    Tuple of (rows, columns) for the layout of the histograms.
bins : integer or sequence, default 10
    Number of histogram bins to be used. If an integer is given, bins + 1
    bin edges are calculated and returned. If bins is a sequence, gives
    bin edges, including left edge of first bin and right edge of last
    bin. In this case, bins is returned unmodified.
kwds : Keyword Arguments
    All other plotting keyword arguments to be passed to
    matplotlib's boxplot function.

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

See Also
--------
matplotlib.axes.Axes.hist : Plot a histogram using matplotlib.

Examples
--------

.. plot::
    :context: close-figs

    >>> df = pd.DataFrame({
    ...     'length': [ 1.5, 0.5, 1.2, 0.9, 3],
    ...     'width': [ 0.7, 0.2, 0.15, 0.2,  1.1]
    ...     }, index= ['pig', 'rabbit', 'duck', 'chicken', 'horse'])
    >>> hist = df.hist(bins=3)

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

Docstring for "pandas.DataFrame.hist" correct. :)
```
Fixed all isues (except the `kwargs`, as it'd fail the validation), and ammended the commit.

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```diff
@@            Coverage Diff             @@
##           master   #20113      +/-   ##
==========================================
- Coverage   91.72%    91.7%   -0.03%     
==========================================
  Files         150      150              
  Lines       49156    49156              
==========================================
- Hits        45090    45078      -12     
- Misses       4066     4078      +12
```

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| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20113/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.84% <0%> (ø)` | :arrow_up: |
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------

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