DOC: Clean up of DataFrame.ewm
- [ ] closes #31647
Though, I don't know what @MarcoGorelli meant in the 5'th issue.

Didn't know how to expand my previous Pull Request: #32212, as I have already deleted the files on my machine.

Here are the screenshots, @WillAyd:

![image](https://user-images.githubusercontent.com/12001304/75212620-caa16e00-5787-11ea-9bd9-532f557d3e33.png)

![image](https://user-images.githubusercontent.com/12001304/75213997-50271d00-578c-11ea-82e0-5f5b986ede6d.png)


In the first image a controversial change might be: log(0.5) -> -ln(2).
The Notes have been expanded with a simple example from the provided link.