Document reference sequence APIs

[jeromekelleher]

#1944 added the basics for reference sequence APIs in Python, but didn't document then.

When we've used them for a little bit and thought some more about how we might use metadata, and the semantics of the URL field, we should document then.

[jeromekelleher]

We had to do basic documentation for this in the initial release in #1985, so going to close this issue with that PR and will open more specific issues later.