clarify IfcPointByDistanceExpression.OffsetVertical, -Lateral, -Longitudinal

1. The documentation describes for OffsetVertical "default" but what is "not default"?
2. There are at least 2 directions which fits the definition: z>0 and z <0
(singular case z=0 is out of the domain scope I think)

**Suggestion: "OffsetVertical is offset in the direction of Axist of IfcAxis2PlacementLinear this instance of IfcPointByDistanceExpression referred from.**

3. OffsetLateral describes "default" but what is "not default"?
4. Described direction is not well-formed

**Suggestion: "OffsetLateral is offset in the direction (Axis vector-cross RefDirection) of IfcAxis2PlacementLinear this instance of IfcPointByDistanceExpression referred from**

**Suggestion: "OffsetLongitudinal" is offset in the direction of local X axis of IfcAxis2PlacementLinear this instance of IfcPointByDistanceExpression referred from**

Start with the IFC4.x specification definitions for IfcPointByDistanceExpression and IfcAxis2PlacementLinear, focusing on OffsetVertical, OffsetLateral, and OffsetLongitudinal. Compare the current direction wording with the suggested axis relationships; done means the default cases and offset directions are unambiguous for all three attributes.

Written by the indexing model from the issue text.