Reword documentation to provide clarity on whether or not IfcExternalSpatialStructureElement is hierarchical

[Moult]

On the IfcSpatialElement page it explains how IfcSpatialStructureElement is hierarchical and IfcSpatialZone is not, but does not mention IfcExternalSpatialStructureElement.

I assume IfcExternalSpatialStructureElement is intended to be hierarchical, but it would be good to make it clear somewhere.

[aothms]

I agree. Hierarchical. From the description "kind of external spaces, regions, and volumes" I would see nothing that forbids hierarchies even if use cases for them are probably hard to find.