DOC: update docstring of pandas.Series.add_prefix 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_prefix) #####################
################################################################################

Concatenate prefix string with panel items names.

Parameters
----------
prefix : str
    The string to add before each item name.

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

See Also
--------
pandas.Series.add_suffix: Add a suffix string to panel items names.

Examples
--------
>>> s = pd.Series([1,2,3,4])
>>> s
0    1
1    2
2    3
3    4
dtype: int64
>>> s.add_prefix('item_')
item_0    1
item_1    2
item_2    3
item_3    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.
Docstring updated, thanks for the comments! # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20313?src=pr&el=h1) Report
> Merging [#20313](https://codecov.io/gh/pandas-dev/pandas/pull/20313?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bfe6ebcb0791e0de06e70cf2dd91129bbacf583d?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/20313/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20313?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20313      +/-   ##
==========================================
+ Coverage    91.7%   91.73%   +0.02%     
==========================================
  Files         150      150              
  Lines       49165    49168       +3     
==========================================
+ Hits        45087    45102      +15     
+ 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/20313?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20313/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.85% <ø> (ø)` | :arrow_up: |
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/20313/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.17% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20313/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.18% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20313/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.66% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20313/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.81% <0%> (+1.73%)` | :arrow_up: |
| [pandas/core/arrays/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20313/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2Jhc2UucHk=) | `76.74% <0%> (+2.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20313?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/20313?src=pr&el=footer). Last update [bfe6ebc...6d599ad](https://codecov.io/gh/pandas-dev/pandas/pull/20313?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 LGTM other than those two minor issues. @TomAugspurger @datapythonista, if it's all fine in here now I'll proceed to modify #20315 (otherwise I'll update this one first) Looks great! Just added a tiny commit adjusting the spacing. Thanks!