DOC: explain EWM
- [ ] closes #34867
- [ ] tests  passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew - Made docs more clear and renamed files

Hello @KrishnaSai2020! 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 2020-06-20 14:03:50 UTC right I'll fix that pep 8 issue and reopen this pr thanks for the PR @KrishnaSai2020 

can you pls fix it though? Not sure about what the previous contributor did, but unlikely his/her PR got merged with the PEP8 issue. 

Looks like your expanded `EM` to `Exponential Weighted` caused it. Ah I see! should i close this, fix and reopen or is there an easier way to edit the PR?? you can fix it in this PR, no need to close it and reopen a new one, thanks! @charlesdong1991  fixed the pep 8 problem I _think_ what was meant by @jreback is to rename `EWM` to `ExponentialMoving` or `ExpontentialMovingWindow` other than to `EWF`.

so you might also need to replace all your `EWF` to the new class name you choose. @charlesdong1991 I think that's all instances refactored let me know if there more. https://github.com/pandas-dev/pandas/pull/34910
