Clarify handling of cycles in Extends for EntityContainers

We do not explicitly forbid cycles in the Extends attribute, i.e. service A extends service B which extends service A.

Which does not seem to be a problem: Extends processing just has to stop when a container is reached that has already been processed because there's nothing new to add to the initial container. In the A extends B extends A example both services will publish the same entity sets etc.