Confusing username availability API

Nobody has claimed this yet.

The API to check availability of a username ([`GET /_matrix/client/r0/register/available`][api]) communicates the availability by either returning `200 Success` or `400 Bad Request` with an error code.

However, the response body in a `200 Success` return also contains an object wrapping a boolean `available` value. This value is described in the spec as always being set to `true` when returned in a 200 response. Which in effect means "this is a constant true value", because the `available` boolean does not exist on non-200 responses. So it doesn't really fill any purpose.

This feels a bit confusing. Adding to the confusion is the fact that when a username is taken the API responds by telling the caller "your API request was invalid", when that shouldn't be the case. Asking if the username "Foo" is taken is a valid request, even if the username happens to be taken (in my mind, at least).

Maybe this can be changed to be less confusing? Changing what it returns would likely be a breaking change though.

[api]: https://matrix.org/docs/spec/client_server/r0.4.0.html#get-matrix-client-r0-register-available

Start with issue 465 and the Matrix Client-Server r0.4.0 specification section for GET /_matrix/client/r0/register/available. Review the existing 200 and 400 response definitions and determine the intended semantics before proposing a change. Done means the behavior is agreed and the affected specification text consistently documents it.

Written by the indexing model from the issue text.