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. 
Hello @Moisan! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/series.py`](https://github.com/Moisan/pandas/blob/5d6d5ed08a6ec75169ecd5df8dba360a6fe41264/pandas/core/series.py) !



##### Comment last updated on September 18, 2018 at 01:46 Hours UTC Passed on travis. Merging.