DOC: update the pandas.DataFrame.pct_change 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 <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>`
- [ ] 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.pct_change)  ###################
################################################################################

Percentage change between the current and previous element.

This is useful in comparing the percentage of change in a time series
of elements.

Parameters
----------
periods : int, default 1
    Periods to shift for forming percent change.
fill_method : str, default 'pad'
    How to handle NAs before computing percent changes.
limit : int, default None
    The number of consecutive NAs to fill before stopping.
freq : DateOffset, timedelta, or offset alias string, optional
    Increment to use from time series API (e.g. 'M' or BDay()).
**kwargs : mapping, optional
    Additional keyword arguments are passed into
    `DataFrame.shift` or `Series.shift`.

Returns
-------
chg : Series or DataFrame
    The same type as the calling object.

Examples
--------
See the percentage change in a Series.

>>> s = pd.Series([90, 91, 85])
>>> s
0    90
1    91
2    85
dtype: int64
>>> s.pct_change()
0         NaN
1    0.011111
2   -0.065934
dtype: float64

See the percentage change in a Series where filling NAs with last
valid observation forward to next valid.

>>> s = pd.Series([90, 91, None, 85])
>>> s
0    90.0
1    91.0
2     NaN
3    85.0
dtype: float64
>>> s.pct_change(fill_method='ffill')
0         NaN
1    0.011111
2    0.000000
3   -0.065934
dtype: float64

Percentage change in French franc, Deutsche Mark, and Italian lira from
1 January 1980 to 1 March 1980.

>>> df = pd.DataFrame({
...     'FR': [4.0405, 4.0963, 4.3149],
...     'GR': [1.7246, 1.7482, 1.8519],
...     'IT': [804.74, 810.01, 860.13]},
...     index=['1980-01-01', '1980-02-01', '1980-03-01'])
>>> df
                FR      GR      IT
1980-01-01  4.0405  1.7246  804.74
1980-02-01  4.0963  1.7482  810.01
1980-03-01  4.3149  1.8519  860.13
>>> df.pct_change()
                  FR        GR        IT
1980-01-01       NaN       NaN       NaN
1980-02-01  0.013810  0.013684  0.006549
1980-03-01  0.053365  0.059318  0.061876

Percentage of change in GOOG and APPL stock volume.

>>> df = pd.DataFrame({
...     '2016': [1769950, 30586265],
...     '2015': [1500923, 40912316],
...     '2014': [1371819, 41403351]},
...     index=['GOOG', 'APPL'])
>>> df
          2016      2015      2014
GOOG   1769950   1500923   1371819
APPL  30586265  40912316  41403351
>>> df.pct_change(axis='columns')
      2016      2015      2014
GOOG   NaN -0.151997 -0.086016
APPL   NaN  0.337604  0.012002

See Also
--------
DataFrame.diff : see the difference of two columns
Series.diff : see the difference of two columns

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

Errors found:
	Errors in parameters section
		Parameters {'kwargs'} not documented
		Unknown parameters {'**kwargs'}
```
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20310?src=pr&el=h1) Report
> Merging [#20310](https://codecov.io/gh/pandas-dev/pandas/pull/20310?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7169830040255c1073f99e2ee8817596a67b1cd5?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20310/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20310?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20310      +/-   ##
==========================================
+ Coverage    91.7%   91.72%   +0.02%     
==========================================
  Files         150      150              
  Lines       49165    49165              
==========================================
+ Hits        45087    45099      +12     
+ Misses       4078     4066      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.11% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.86% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20310?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20310/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.85% <100%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20310/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.81% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20310?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20310?src=pr&el=footer). Last update [7169830...a0dcac2](https://codecov.io/gh/pandas-dev/pandas/pull/20310?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Made some touchups. Thanks @myles! @TomAugspurger small comment on when merging: can you remove all the different commit messages (in case of those DOC PRs it's mostly useless to keep them). It's maybe not that important and only my nitpicky self, but it's only a "click in box, ctrl-A, del" combo away :-) 👍 