DOC PR07: Add description to parameter skipna
- [ ] closes https://github.com/pandanistas/pandanistas_sprint_jakarta2020/issues/18
- [ ] tests added / passed
- [ ] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

output of `python scripts/validate_docstrings.py pandas.Series.argmax`:
```
################################################################################
################################## Validation ##################################
################################################################################

4 Errors found:
        No extended summary found
        Parameters {'*args', '**kwargs'} not documented
        Missing description for See Also "numpy.ndarray.argmax" reference
        No examples section found
```
