DOC: update the pandas.Series.add_suffix docstring

[astrastefania]

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:

################################################################################
##################### Docstring (pandas.Series.add_suffix) #####################
################################################################################

Add a suffix string to panel items names.

Parameters
----------
suffix : str
    The string to add after each item name.

Returns
-------
Series
    Original Series with updated item names.

See Also
--------
pandas.Series.add_prefix: Concatenate prefix string with panel items names.

Examples
--------
>>> s = pd.Series([1,2,3,4])
>>> s
0    1
1    2
2    3
3    4
dtype: int64
>>> s.add_suffix('_item')
0_item    1
1_item    2
2_item    3
3_item    4
dtype: int64

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

Errors found:
	No extended summary 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.

[datapythonista]

Same comments as in #20313 :)

[astrastefania]

Docstring fixed. :)

[datapythonista]

Didn't check, but as this is defined in generic.py it's likely to be reused by both Series and DataFrame. Can you build the whole documentation and check the page for DataFrame.add_sufix please? If it says that it returns a Series, and all the other stuff specific to Series it should be changed, so both pages are correct.

[codecov]

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@@            Coverage Diff            @@
##             master   #20315   +/-   ##
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Flag	Coverage Δ
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Impacted Files	Coverage Δ
pandas/core/generic.py	95.85% <ø> (ø)

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[astrastefania]

@datapythonista, @TomAugspurger mentioned in #20313 this edited docstring in core/generic.py it will be used by both Series.add_suffix and DataFrame.add_suffix to avoid splitting the docstring.

If all agree both #20315 and #20313 should be fine now. 😸

[datapythonista]

As it's for both Series and DataFrame would make sense to have the .add_prefix method of both classes. Or none of them, there is some discussion about whether to have them or not here: #20289 (comment)

[TomAugspurger]

Good call. Added in the latest commit to both.

[TomAugspurger]

Good call. Added in the latest commit to both.

[TomAugspurger]

Thanks again @astrastefania :)