IfcPositioningElement documentation errors

These issues related primarily to [IfcPositioningElement](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcPositioningElement.htm)

1. The first sentence should be revised to "An abstract entity definition for positioning and annotating elements that are used to position other elements relatively.". The word "New" is unnecessary. At what point does this entity cease to be "new"? The change log indicates IfcPositionElement was new in IFC 4.1
2. The abstract icon is missing
3. The formal proposition clearly states that `ObjectPlacement` is required for grid. However, when defined at this level, it is applied to all subtypes. This has led to an invalid implementation in the validation service. IfcReferent without ObjectPlacement are flagged as an error. IfcReferent is not a subtype of IfcGrid, therefore the formal proposition should not apply.

Read the linked IfcPositioningElement definition and its change log first. Check the requested sentence, abstract icon, and ObjectPlacement proposition against the IfcReferent and IfcGrid subtype relationships and the reported validation behavior. Done means all three documentation or specification errors are corrected without incorrectly applying the proposition to IfcReferent.

Written by the indexing model from the issue text.