* Use the terminology 'login type' everywhere instead of mixing up 'stage type'
and 'login type'
* Don't have a separate 'APIs using the User-Interactive Authentication
mechanism' section, because (a) it doesn't make much sense to organise the
APIs this way, and (b) it was a set of lies anyway.
* Move '/account/password' definition into registration.yaml so that register
and password can share a section in the spec; remove duplicate doc for
/password.
* Write some words on using 3pids for /login