DOC: update the Seriex.idxmax and Series.idxmin 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:

```
Errors found:
	Errors in parameters section
		Parameters {'args', 'kwargs'} not documented
		Unknown parameters {'*args, **kwargs :'}
		Parameter "*args, **kwargs :" has no type
```

Parameters args and kwargs don't need types, put them on one line to keep it concise.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20174?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@080ef0c`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20174/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20174?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20174   +/-   ##
=========================================
  Coverage          ?    91.7%           
=========================================
  Files             ?      150           
  Lines             ?    49156           
  Branches          ?        0           
=========================================
  Hits              ?    45078           
  Misses            ?     4078           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.08% <100%> (?)` | |
| #single | `41.85% <57.14%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20174?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20174/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.84% <ø> (ø)` | |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/20174/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.86% <100%> (ø)` | |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20174/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.18% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20174?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/20174?src=pr&el=footer). Last update [080ef0c...41394ae](https://codecov.io/gh/pandas-dev/pandas/pull/20174?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
