* Make the purpose of the `auth` key in /register requests explicit, and say
that it should be empty at first.
* Restructure the UA-auth section a bit.
* In the UA-auth section, say that clients should submit no `auth` to start
with, and add 'Stage 0' representing this to the example.
* s/{stage,login} type/authentication type/ in the UA-auth section. Seems
clearer to me.
* Try to distinguish the example responses from the example requests by giving
an HTTP header.
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