DOC: Fix Series nsmallest and nlargest docstring/doctests
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Based on #22459. Fix the docstring for Series.nsmallest and Series.nlargest. I did both together since the same example could be used. I removed both from the skipped doctests in `ci/doctests.sh`.

I also found out about the "all" option for the `keep` parameter which was surprisingly not documented. 