DOC: Clarify where to the additional arguments for some win_types
For example, std needs to specify when win_types is gaussian. However, std should be specified in the operation argument, not as one of the rolling arguments. This change is to clarify this point. 

- [ ] closes #34593
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @wchen928! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-06-17 20:58:13 UTC Merged master to fix the CI failure. Thanks @wchen928 