ENH: `Styler.apply` accept ndarray return with `axis=None` for consistency 
- [x] closes #39359
- [x] tests added / passed
- [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
- [x] whatsnew entry

Also minor edits to docs to extend information and show examples.

cc @TomAugspurger 