DOC: Clarify rolling min_periods default value GH21489
- [ ] closes #21489
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew Clarifies the documentation of the default value for the min_periods argument of the rolling function.

Hello @mattboggess! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/window.py`](https://github.com/mattboggess/pandas/blob/ae05091bd4a3c287c4466f43ba841ea65f46e296/pandas/core/window.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23021?src=pr&el=h1) Report
> Merging [#23021](https://codecov.io/gh/pandas-dev/pandas/pull/23021?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5551bcf9d297ea8a0aeffb70b17ae6730e8abf89?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23021/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23021?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23021   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files         169      169           
  Lines       50835    50835           
=======================================
  Hits        46868    46868           
  Misses       3967     3967
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `42.35% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23021?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/23021/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.4% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23021?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/23021?src=pr&el=footer). Last update [5551bcf...9323c00](https://codecov.io/gh/pandas-dev/pandas/pull/23021?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 this is fine. thank you.