fix is_scalar documentation (#28998)
- [x] closes #28998
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

is_scalar documentation was wrong, see #28998
I was about to do the same... ^^ @WillAyd no harm in merging to master since this is going in 1.0. Thanks @andreasbuhr!