DOC: Extend documentation on RandomVariables

[ricardoV94]

Idea or request for content:

I currently wrote a Gist just to investigate how updates work, that I think would be nice to add to the official docs. It needs to be extended slightly on the Scan part, and perhaps also add one on cloning graphs and the issue with default_updates

https://gist.github.com/ricardoV94/7aa6c23726f20fbec95a1cda449c15f5