Categorical doc fixups
Minor doc updates as part of #8375
- docstring in in cut/qcut + docstring examples #8077 (comment)
- whatsnew: In the note change "levels" to "categories" and s/Schultz/Schulz/

@jorisvandenbossche can you have a look at the docstring fixes?
 yes, looked at them and looking good!
 thanks!
