True north should be specified at where it is sampled

Originally something I discussed here: https://forums.buildingsmart.org/t/how-is-true-north-and-magnetic-north-stored-in-ifc-and-how-does-it-interact-with-grid-north/3491

When a map conversion is supplied, the true north becomes a calculated value at a particular easting and northing. For example, at Easting X and Northing Y, the true north vector is (A,B).

The current description of true north has this sentence:

> NOTE If a geographic placement is provided using [IfcMapConversion](http://localhost:5000/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMapConversion.htm) then the true north is for information only. In case of inconsistency, the value provided with [IfcMapConversion](http://localhost:5000/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMapConversion.htm) shall take precedence.

I propose this modification to the description:

> NOTE If a geographic placement is provided using [IfcMapConversion](http://localhost:5000/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMapConversion.htm) then the true north is for information only, **as calculated at the eastings and northings of the project origin**. In case of inconsistency, the value provided with [IfcMapConversion](http://localhost:5000/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMapConversion.htm) shall take precedence.

Ping @LeeGregory12d @SergejMuhic