Not clearly explained: entityReferenceType, EntityReferenceTypeVocabulary

[IgorRodchenkov]

BioPAX L3 OWL comments regarding the entityReferenceType
property and EntityReferenceTypeVocabulary
and the documentation are not clear.
It mentions about grouping, such as by homology or function, but does not explain when and why to use this property and vocabulary terms.

Perhaps, this property and corresponding controlled vocabulary are only useful with generic entity references, i.e., ones that have not empty memberEntityReference property. Let's make it clear.
If there are other use cases, let's include them into the rdfs:comment as well.