Document the behavior of `axis=None` with `style.background_gradient`
Add docstring entry for using `axis=None`with `style.background_gradient`, which was added in #21259. Also reword to be in numerical order and consistent with how `highlight_max` and `highligh_min` defines column-wise and row-wise.

- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
