DOC/REF: Rolling/Expanding/EWM docstrings
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them


- Create a new `window/doc.py` file for common template
- Using `@doc` decorator instead of `@Appender`
- Standardize descriptions and sections

<img width="1255" alt="Screen Shot 2021-01-16 at 5 06 52 PM" src="https://user-images.githubusercontent.com/10647082/104828385-c7601680-5860-11eb-99b5-e73357719348.png">
 
