DOC: update the DataFrame.at[] docstring

[akosel]

Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

• PR title is "DOC: update the docstring"
• The validation script passes: scripts/validate_docstrings.py <your-function-or-method>
• The PEP8 style check passes: git diff upstream/master -u -- "*.py" | flake8 --diff
• 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:

# 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.

[WillAyd]

Very minor edits otherwise looks good

[WillAyd]

For the See Also section the format should be item : description so move to the description up to the same line and just break where length exceeds 75 characters

[WillAyd]

Capitalize Series

[jorisvandenbossche]

Apart from the comments of @WillAyd, looks good!

[akosel]

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]

Codecov Report

Merging #20290 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            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%)	⬆️
#single	41.86% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/indexing.py	93.02% <ø> (ø)	⬆️
pandas/plotting/_converter.py	66.81% <0%> (+1.73%)	⬆️

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[TomAugspurger]

Thanks @akosel!