IfcGeotechnicalStratum is abstract

The entity seems to have remained abstract even with the deletion of its subtypes.

EDIT: PredefinedType attribute documentation cannot be accessed?

(another) EDIT: PredefinedType is not listed in the markdown. This is not consistent across the entities.

(hope final) EDIT: WR CorrectPredefinedType does not have documentation.

(and another) EDIT: for entities with PredefinedType but no `IfcTypeObject` the WR HasObjectType is assigned. The rule looks semantically the same (apart from one detail) but has a bit of a different documentation. The documentation seems a bit of in regards to the rule but maybe @TLiebich knows more about the rationale. It seems to be only in IFC4 entities.