Document expansion options

@nickm, I hope this is comprehensible. I did manage to add an example to the docs, and of course there are some test/examples.

I'm going to "request review" mostly to ask you if you're happy with the way things are here. If you dislike any of the syntax or names or things I've invented here please do say - ideally, by opening a ticket, which I will then treat as a release blocker. I'm open to alternative suggestions.

Obviously ordinary review comments are welcome too :-).

Completes #16 (closed). Helps a lot with #12 (closed). Fixes #11 (closed) (depends on the template author using the new expect feature)