Fix register nym docs

The cost of documentation becomes clear immediately in 9272fad.

Other than that I'm wondering how far I should go explaining the returned message.
The hashing and signing part is true for *all* received messages (and sub-messages as far as I can tell). The `@createUserAccount` message type is the relevant info here.

I'm still for leaving it in at the moment and reorganizing as we start to puzzle things together.