Update nullable integer docs with None instead of np.nan
Update nullable integer docs with None instead of np.nan

- [X] closes #29599 
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This seems fine.

I'm not sure whether avoiding the original problem (a trip through a float dtype ndarray) is worth fixing or not, when the user provides `np.nan` (which is a float). thanks @lucassa3  @jreback no problem. Glad I could help ^^