[Task] Document semantics of recursion in Aspect Models

[atextor]

Is your task related to a problem? Please describe.
It is possible to create recursive structures in Aspect Models when there are cycles in the graph of elements. For example, an Entity could have a Property with a samm-c:SingleEntity Characteristic pointing back to the Entity. In order to construct JSON payloads that correspond to the Aspect Model, this is only allowed when there is a "recursion breaker": Somewhere along the chain that constitutes the cycle in the model there needs to a samm-c:Either Characteristic or an optional Property. In the formal specification (SHACL shapes) this is considered and in the PropertyTest there is a corresponding test. However, there exists no sufficient textual description of this in the specification.

This was encountered here: eclipse-esmf/esmf-aspect-model-editor#50

Describe the solution you'd like
Document semantics of allowed and disallowed recursion