Reword schema intro docs

E.g outdated:

6.1 IfcSharedBldgElements

The shared building elements (IfcSharedBldgElements) define the subtypes of IfcBuildingElement, which is defined in the IfcProductExtension. Those subtypes are the major elements, which constitutes the architectural design of the building structure.

The elements (e.g. wall, beam, column, slab, roof, stair, ramp, window, door and covering) are the main components of the raw building (or carcass) which is central for the exchange of project data. For each of those elements a geometric use definition is provided, which introduces the correct application of available shape representation types for the elements.