DOC: update the pandas.Series.add_suffix 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.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.
Same comments as in #20313 :) Docstring fixed. :) # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20315?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@7c5bae3`). [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/20315/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20315?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20315   +/-   ##
=========================================
  Coverage          ?   91.73%           
=========================================
  Files             ?      150           
  Lines             ?    49174           
  Branches          ?        0           
=========================================
  Hits              ?    45108           
  Misses            ?     4066           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.11% <ø> (?)` | |
| #single | `41.86% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20315?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20315/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.85% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20315?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/20315?src=pr&el=footer). Last update [7c5bae3...fdba3d4](https://codecov.io/gh/pandas-dev/pandas/pull/20315?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @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. 😸  Thanks again @astrastefania :)