DOC: fix mistake in Series.str.cat
Fix error in API-docstring that was introduced at the end of #20347 due to timepressure for the v.0.23-cutoff: removed functionality that categorical callers get categorical output, but forgot to adapt doc-string.

Unfortunately, this survived both #20347 and the follow-up, but since v.0.23.1 is coming soon, I didn't wanna let this opportunity pass.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21330?src=pr&el=h1) Report
> Merging [#21330](https://codecov.io/gh/pandas-dev/pandas/pull/21330?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7dc6f7023ce439e8ef10ef336f314b10192911c5?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/21330/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/21330?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21330   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files         153      153           
  Lines       49549    49549           
=======================================
  Hits        45512    45512           
  Misses       4037     4037
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.25% <ø> (ø)` | :arrow_up: |
| #single | `41.87% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21330?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/21330/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.62% <ø> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/21330/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.12% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21330?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/21330?src=pr&el=footer). Last update [7dc6f70...439021e](https://codecov.io/gh/pandas-dev/pandas/pull/21330?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @h-vetinari Thanks!