Make Rolling.apply documentation clearer
Fix #25656 by making Rolling.apply documentation clearer (hope this is what @js3711 intended)
- [x] closes #25656 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25712?src=pr&el=h1) Report
> Merging [#25712](https://codecov.io/gh/pandas-dev/pandas/pull/25712?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/69ae24bf5de3e7c45fe32f5f17728bacc0bceaef?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/25712/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25712?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25712   +/-   ##
=======================================
  Coverage   91.24%   91.24%           
=======================================
  Files         172      172           
  Lines       52967    52967           
=======================================
  Hits        48332    48332           
  Misses       4635     4635
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.82% <ø> (ø)` | :arrow_up: |
| #single | `41.74% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25712?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/25712/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/25712?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/25712?src=pr&el=footer). Last update [69ae24b...d9439f6](https://codecov.io/gh/pandas-dev/pandas/pull/25712?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25712?src=pr&el=h1) Report
> Merging [#25712](https://codecov.io/gh/pandas-dev/pandas/pull/25712?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/69ae24bf5de3e7c45fe32f5f17728bacc0bceaef?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/25712/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25712?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25712   +/-   ##
=======================================
  Coverage   91.24%   91.24%           
=======================================
  Files         172      172           
  Lines       52967    52967           
=======================================
  Hits        48332    48332           
  Misses       4635     4635
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.82% <ø> (ø)` | :arrow_up: |
| #single | `41.74% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25712?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/25712/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/25712?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/25712?src=pr&el=footer). Last update [69ae24b...d9439f6](https://codecov.io/gh/pandas-dev/pandas/pull/25712?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @Bharat123rox 