DOC/REF: Rolling/Expanding/EWM docstrings

[mroeschke]

• Ensure all linting tests pass, see here 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

[jreback]

cc @jorisvandenbossche @simonjayhawkins if you have any comments

[simonjayhawkins]

OT i don't think *args should be used after the keyword argument ddof, mypy normally complains about the function receiving multiple values for argument.

[mroeschke]

I might need a precursor PR to align function signatures.

@jreback thoughts on all functions having a signature of def func(*args, <keyword_only_arguments>, **kwargs)?

Though *args, **kwargs may never be used.

EDIT: Nevermind

[jreback]

hmm somthing going wrong on the docs build

[mroeschke]

Fixed the doc build failing tests and all green.

Refactored now to omit empty sections.

[jreback]

ahh can you rebase

[jreback]

cc @simonjayhawkins @jorisvandenbossche if any other comments.

[simonjayhawkins]

Thanks @mroeschke lgtm.

[mroeschke]

Going to merge and can address any follow ups in another PR