DOC: some clean-up of the apply docs (follow-up #18577)
Follup-up on #18577

cc @jreback (instead of yet another round of review, this will go a bit quicker)
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19573?src=pr&el=h1) Report
> Merging [#19573](https://codecov.io/gh/pandas-dev/pandas/pull/19573?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6b0c7e72b141831b7a9a5651f9e19eef53ec9e76?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/19573/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19573?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19573   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files         150      150           
  Lines       48781    48781           
=======================================
  Hits        44695    44695           
  Misses       4086     4086
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.99% <ø> (ø)` | :arrow_up: |
| #single | `41.73% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19573?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/sparse/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19573/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL2ZyYW1lLnB5) | `94.81% <ø> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19573/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.42% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19573?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/19573?src=pr&el=footer). Last update [6b0c7e7...552cc43](https://codecov.io/gh/pandas-dev/pandas/pull/19573?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jorisvandenbossche 