Add direct reference in IfcBuildingElementProxy to use IfcBuiltElement as an arbitrary built element

Greetings!

There is a note in IfcBuiltElement specifying that "_It is advised not to use [IfcBuildingElementProxy](https://standards.buildingsmart.org/IFC/RELEASE/IFC4_3/HTML/lexical/IfcBuildingElementProxy.htm) to represent an arbitrary built element, but to use a direct instantiation of [IfcBuiltElement](https://standards.buildingsmart.org/IFC/RELEASE/IFC4_3/HTML/lexical/IfcBuiltElement.htm) instead._".

Since the IfcBuildingElementProxy was used as a general built element until IFC version 4.3, adding a direct reference to this change in its documentation — perhaps using the same note found in IfcBuiltElement — could help users easily identify this update.

Additionally, are there any uses for IfcBuildingElementProxy that cannot be accomplished with a direct instantiation of IfcBuiltElement?

Start by comparing the IfcBuildingElementProxy and IfcBuiltElement documentation linked in the issue, then locate their corresponding documentation or schema entries in the repository. Add a direct reference explaining the IFC 4.3 change, and document or resolve the question about remaining uses of IfcBuildingElementProxy.

Written by the indexing model from the issue text.