Improve HttpResponse.OnCompleted and RegisterForDispose XML docs

Question that came up about OnCompleted:

> Is HttpResponse::OnCompleted guaranteed to be called at the end of request processing including all failure scenarios such as connection drop by client, server side request processing timeout, client side request processing timeout, etc.

Improve XML docs to clarify that the `OnCompleted` callback is always executed or RegisterForDispose always disposes the registered object.

Can also update `IHttpResponseFeature.OnCompleted`