Docs: Give clearer example of multiple query predicates in one ql file

[owen-mc]

trafficstars

The new names aren't great, so feel free to change them, but I think we do need an explicit example of updating two relations using one ql file.

[owen-mc]

Another suggestion for improvement: link to some actual upgrade (and downgrade) scripts, so it's easy to see the kinds of things that people do and how they do them. And perhaps explicitly mention that if you want to create a new table with nothing in it then you don't have to do anything.