Fix multi-line returns sections in docs

[T-Nicholls]

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 (sphinx-contrib/napoleon#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.