DOC: update the DataFrame.eval() 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:

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

Evaluate a string describing operations on DataFrame columns.

Operates on columns only, not specific rows or elements.  This allows
`eval` to run arbitrary code, which can make you vulnerable to code
injection if you pass user input to this function.

Parameters
----------
expr : str
    The expression string to evaluate.
inplace : bool, default False
    If the expression contains an assignment, whether to perform the
    operation inplace and mutate the existing DataFrame. Otherwise,
    a new DataFrame is returned.

    .. versionadded:: 0.18.0.
kwargs : dict
    See the documentation for :func:`~pandas.eval` for complete details
    on the keyword arguments accepted by
    :meth:`~pandas.DataFrame.query`.

Returns
-------
ndarray, scalar, or pandas object
    The result of the evaluation.

See Also
--------
DataFrame.query : Evaluates a boolean expression to query the columns
    of a frame.
DataFrame.assign : Can evaluate an expression or function to create new
    values for a column.
pandas.eval : Evaluate a Python expression as a string using various
    backends.

Notes
-----
For more details see the API documentation for :func:`~pandas.eval`.
For detailed examples see :ref:`enhancing performance with eval
<enhancingperf.eval>`.

Examples
--------
>>> df = pd.DataFrame({'A': range(1, 6), 'B': range(10, 0, -2)})
>>> df
   A   B
0  1  10
1  2   8
2  3   6
3  4   4
4  5   2
>>> df.eval('A + B')
0    11
1    10
2     9
3     8
4     7
dtype: int64

Assignment is allowed and by default the original DataFrame is not
modified.

>>> df.eval('C = A + B')
   A   B   C
0  1  10  11
1  2   8  10
2  3   6   9
3  4   4   8
4  5   2   7
>>> df
   A   B
0  1  10
1  2   8
2  3   6
3  4   4
4  5   2

Use inplace=True to modify the original DataFrame.

>>> df.eval('C = A + B', inplace=True)
>>> df
   A   B   C
0  1  10  11
1  2   8  10
2  3   6   9
3  4   4   8
4  5   2   7

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

Docstring for "pandas.DataFrame.eval" correct. :)
```
On the beggining of the text of your PR, there is a space when using `[x ]`, there should be no space between 'x' and ']', as it leads to uncorrect formatting. Fixed. All fixes made, thanks for comments. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20209?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@faba7ef`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20209/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20209?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20209   +/-   ##
=========================================
  Coverage          ?    91.7%           
=========================================
  Files             ?      150           
  Lines             ?    49165           
  Branches          ?        0           
=========================================
  Hits              ?    45087           
  Misses            ?     4078           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.09% <ø> (?)` | |
| #single | `41.86% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20209?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20209/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.18% <ø> (ø)` | |

------

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