Allows Docstring to return a list of DocstringReturns to better work with numpydoc

[mayitbeegh]

PR for #33
Numpydoc supports multiple return values in the return section. Here's an example:

Returns
-------
err_code : int
    Non-zero value indicates error code, or zero on success.
err_msg : str or None
    Human readable error message, or None on success.

This PR attempts to add support for multiple return values, since the library only supports a single return value at the moment.
The ideal solution would be updating the existing Docstring.returns to return T.List[DocstringReturns] instead of T.Optional[DocstringReturns]. This is a breaking change, but it's more consistent with how params work. The alternative solution is adding a new property that returns T.List[DocstringReturns], which is what's in this draft PR. I also added a few lines of unit test to showcase the use case. This is only a draft PR. Depending on how the maintainers feel about the backwards compatibility, things may go very differently.

[rr-]

Props for not breaking backwards compatibility, plus I feel like having a separate returns for a single property is a good option since most of the codebases I'd worked with do not document alternative return values this way.

LMK if you're OK with merging and releasing.

[mayitbeegh]

Thanks @rr-, I'd like to add more unit test code to keep up with the rest of the project.

[mayitbeegh]

Done.

[rr-]

Thank you; released as 0.9