DOC: Correct docstring for Series.apply
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

Ref: https://github.com/pandas-dev/pandas/pull/39941#issuecomment-829604511 and the conversation preceding it. While I don't find @MarcoGorelli's suggestion as implemented here very clear, it at least makes it correct and I don't see a better way.