[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.
This isn't a docs comment, but it's a pity that the method for allocating a random number and allocating a specific number pass the private_id parameter in differently