DOC: Removed duplicate returns section
Closes https://github.com/pandas-dev/pandas/issues/20822
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20823?src=pr&el=h1) Report
> Merging [#20823](https://codecov.io/gh/pandas-dev/pandas/pull/20823?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4aac0c8a3ffd841a6916a5b49a8c4aa55fa8b080?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20823/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20823?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20823      +/-   ##
==========================================
- Coverage   91.78%   91.78%   -0.01%     
==========================================
  Files         153      153              
  Lines       49273    49271       -2     
==========================================
- Hits        45224    45222       -2     
  Misses       4049     4049
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.17% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.88% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20823?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/20823/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.29% <ø> (-0.01%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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