Document stubbing services caveat

https://guides.emberjs.com/release/testing/testing-components/#toc_stubbing-services

This section shows how to stub a service in a component test, but there is a caveat. If a service was accessed in an initializer, calling `register` will not work. This should be documented somehow. A workaround would be nice as well, but I don't think there *is* a good workaround at the moment.