DOC: explain EWM

[KrishnaSai2020]

• closes DOC: pandas/pandas/core/window/ewm.py #34867
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[pep8speaks]

Hello @KrishnaSai2020! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

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

Comment last updated at 2020-06-20 20:06:07 UTC

[KrishnaSai2020]

@charlesdong1991 believe this is done please have a look when you can.

[charlesdong1991]

thanks, this looks much cleaner now! only one nitpick

[charlesdong1991]

hey, it's very close.

I think you could search pandas.core.window.ewm in pandas, and there are some other usages of it, probably in some rst files, otherwise doc validation won't pass.

An easier walkaround would be keeping the file name unchanged as ewm.py, the main purpose of this PR i think is to make EMW class name more intuitive, while not too big deal to have ewm.py in dev, but you might still need to change a few places to make CI pass.

[jreback]

thanks @KrishnaSai2020

[KrishnaSai2020]

@jreback np and @charlesdong1991 thanks for your help!