[Reviewer: Ellie] Update API docs to cover specific numbers

Updates the Ellis API docs to cover the feature I added in https://github.com/Metaswitch/ellis/pull/61.

I've also deleted two parts where we described function we hadn't implemented - I thought this was confusing.
Actually, they don't - both accept either URL parameters or form-encoded bodies (but not JSON). The rest of the docs are just wrong. (I'll fix up.)