LRA specification: descriptions for start/end and LRA do not say which response codes are valid

The specification does not specify the HTTP response codes for starting and ending LRAs. The relevant sections of the specification are:

> Performing a `POST` on ... creates an LRA.
> Performing a `PUT` on .../close will trigger the successful completion of the LRA ...

This means that it is vague as to whether asynchronous operations are supported, which they should be (since the target use case for LRAs is a microservice based environment).

The proposal (and reference implementation) should be updated to include the response codes and it SHOULD include the ability to return "202 Accepted"