Reword documentation here and there in IfcRepresentationItem IfcGeometricRepresentationContext, et all

Not a schema change, but I remember when I got started getting my head around IFC's concept of contexts, representations, mapped representations, items, and items within items made me headdesk more than once. Not so much the actual concept - which with the exception of nested mapping (personal opinion here) makes sense, but the way it is described in such technical language in the docs which discourage implementation.

@theoryshaw brings up this nugget:

> A representation item contributes to the definition of another representation item when it is referenced by that representation item.

Here's another by @theoryshaw:

> A representation is a collection of one or more representation items that are related in a specified representation context. The relationship of representation item to representation context is the basis for distinguishing which representation item entities are related.

I propose to make a number of changes that will make this easier to understand, with some real life examples. Purely docstring changes to help implementers.