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.