DOC: update the DataFrame.applymap 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>`
- [x] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```
python scripts/validate_docstrings.py pandas.DataFrame.applymap

################################################################################
#################### Docstring (pandas.DataFrame.applymap)  ####################
################################################################################

Apply a function to a Dataframe elementwise.

This method applies a function to a DataFrame that is intended to
operate elementwise, i.e. like doing `map(func, series)` for each
series in the DataFrame.

Parameters
----------
func : function
    Python function, returns a single value from a single value.

Returns
-------
pandas.DataFrame
    A transformed DataFrame.

See also
--------
DataFrame.apply : Apply a function along input axis of DataFrame

Examples
--------
>>> df = pd.DataFrame([[1.,2.], [3.,4.]])
>>> df
     0    1
0  1.0  2.0
1  3.0  4.0
>>> df = df.applymap(lambda x: x**2)
>>> df
     0     1
0  1.0   4.0
1  9.0  16.0

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

Docstring for "pandas.DataFrame.applymap" correct. :)
```
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20136?src=pr&el=h1) Report
> Merging [#20136](https://codecov.io/gh/pandas-dev/pandas/pull/20136?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/840d4323935348a890d6b868f2b18e8dddd22199?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20136/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20136?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20136      +/-   ##
==========================================
- Coverage   91.72%    91.7%   -0.03%     
==========================================
  Files         150      150              
  Lines       49156    49152       -4     
==========================================
- Hits        45090    45074      -16     
- Misses       4066     4078      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.08% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.84% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20136?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20136/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.18% <ø> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20136/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20136/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `96.78% <0%> (-0.02%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/20136/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.85% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20136/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.66% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/20136/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.06% <0%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20136/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.84% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20136?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20136?src=pr&el=footer). Last update [840d432...26c5aa8](https://codecov.io/gh/pandas-dev/pandas/pull/20136?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @orereta !