Revisit the meaning of aggregate-principal-transformer

Meaning of Elytron aggregate-principal-transformer should be revised. Also one point about regex-validating-principal-transformer is included since it seems its use cases are related to aggregate-principal-transformer. See:

• It seems that it works like "It iterates through assigned Principal Transformers and returns the first non-null transformed Principal" - is it correct and intended behaviour? Is "aggregate-principal-transformer" appropriate name for transformer which works like that?
• What is the use case for regex-validating-principal-transformer. This transformer just checks some pattern and if it does not match then it rewrites Principal name to null. I think it can be useful in aggregate-principal-transformer, when it can check that name matches some pattern in first transformer (regex-validating-principal-transformer) and then transforms principal in another transformer (e.g. constant-principal-transformer). Is there any other use case?
• When can aggregate-principal-transformer return any other Principal Transformer than first of the list? I think only user implemented custom-principal-transformer can currently return null (which enable iterating to another principal transformer in the list). Also regex-validating-principal-transformer can be used for returning non-first transformer, as I mentioned in previous point. Is there any real scenario when aggregate-principal-transformer can be used?

This issue is reported based on previous discussion with engineering.