DOC: clarify default window in rolling method
- [x] closes #17893 
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=h1) Report
> Merging [#18177](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c8aad92c44098caf7d5875bd0803a2abffb3b34a?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18177/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18177      +/-   ##
==========================================
+ Coverage    91.4%   91.41%   +<.01%     
==========================================
  Files         163      163              
  Lines       50068    50068              
==========================================
+ Hits        45764    45768       +4     
+ Misses       4304     4300       -4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.22% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.35% <ø> (-0.05%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.37% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.02% <0%> (-0.02%)` | :arrow_down: |
| [pandas/plotting/\_tools.py](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL190b29scy5weQ==) | `72.92% <0%> (ø)` | :arrow_up: |
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `58.1% <0%> (ø)` | :arrow_up: |
| [pandas/core/config\_init.py](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnX2luaXQucHk=) | `98.34% <0%> (ø)` | :arrow_up: |
| [pandas/io/parquet.py](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnF1ZXQucHk=) | `65.38% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_style.py](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19zdHlsZS5weQ==) | `74.28% <0%> (ø)` | :arrow_up: |
| [pandas/io/msgpack/\_version.py](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree#diff-cGFuZGFzL2lvL21zZ3BhY2svX3ZlcnNpb24ucHk=) | `44.65% <0%> (ø)` | :arrow_up: |
| ... and [6 more](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18177?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/18177?src=pr&el=footer). Last update [c8aad92...7085458](https://codecov.io/gh/pandas-dev/pandas/pull/18177?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I've added a link to the scipy docs - I presumed this is where all the windows come from? Is this a bit better? seems ok to me, @jorisvandenbossche  @jorisvandenbossche  @dstansby Thanks a lot!