Fix multi-line returns sections in docs

Currently, the formatting of some of our returns sections (those that span multiple lines of code) is messed up on read the docs, for an example look at `DataSourceManager.get_fields()`. This is a Sphinx Napoleon issue (https://github.com/sphinx-contrib/napoleon/issues/4); however, we can fix it locally one of two ways:

1. Reduce all our `Returns:` sections to fit on one line, most do already.
2. Change out indentation style to that shown as a fix in the above issue, however for consistency we would probably have to update all other sections to that indentation style too.