DOC: update the DataFrame.at[] 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:

```
# paste output of "scripts/validate_docstrings.py <your-function-or-method>" here
# between the "```" (remove this comment, but keep the "```")
################################################################################
####################### Docstring (pandas.DataFrame.at)  #######################
################################################################################

Access a single value for a row/column label pair.

Similar to ``loc``, in that both provide label-based lookups. Use
``at`` if you only need to get or set a single value in a DataFrame
or Series.

See Also
--------
DataFrame.iat : Access a single value for a row/column pair by integer position
DataFrame.loc : Access a group of rows and columns by label(s)
Series.at : Access a single value using a label

Examples
--------
>>> df = pd.DataFrame([[0, 2, 3], [0, 4, 1], [10, 20, 30]],
...                   index=[4, 5, 6], columns=['A', 'B', 'C'])
>>> df
    A   B   C
4   0   2   3
5   0   4   1
6  10  20  30

Get value at specified row/column pair

>>> df.at[4, 'B']
2

Set value at specified row/column pair

>>> df.at[4, 'B'] = 10
>>> df.at[4, 'B']
10

Get value within a series

>>> df.loc[5].at['B']
4

Raises
------
KeyError
    When label does not exist in DataFrame

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

Errors found:
	No returns section found

```

If the validation script still gives errors, but you think there is a good reason
to deviate in this case (and there are certainly such cases), please state this
explicitly.
Thank you for the comments! I believe I have addressed them all, but please let me know if there's anything else that can be improved. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20290?src=pr&el=h1) Report
> Merging [#20290](https://codecov.io/gh/pandas-dev/pandas/pull/20290?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/df2e361b899d445fe71d4a9188c546f593251cea?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20290/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20290?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20290      +/-   ##
==========================================
+ Coverage    91.7%   91.73%   +0.02%     
==========================================
  Files         150      150              
  Lines       49168    49168              
==========================================
+ Hits        45090    45102      +12     
+ Misses       4078     4066      -12
```

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

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20290?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20290/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.02% <ø> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20290/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/20290?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/20290?src=pr&el=footer). Last update [df2e361...d3f78b4](https://codecov.io/gh/pandas-dev/pandas/pull/20290?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @akosel!