DOC: Fix summaries not ending with a period
I have fixed all 'Summary does not end with a period' issue except these two:
![image](https://user-images.githubusercontent.com/25010571/49705373-ed2f8180-fbeb-11e8-9f7c-5cf8d2b3aa81.png)
I couldn't find the linenumbers in the corresponding file, and also couldn't fix it by looking for 'argmin' or 'argmax' in Panda directory. Please advise if there is anything else I could try.

- [x] closes #24164
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
