No documentation to prohibit multiple shape representations of the same context with different representation types

Originally brought up [here](https://forums.buildingsmart.org/t/is-it-valid-to-mix-representation-item-types/2792)

It is legal for an IFC product to have a product definition shape that has two shape representations that both belong to the exact same geometric context, but have different representation types (e.g. one as a point cloud, one as a brep).

Even though it is legal, is it intended to allow mixing of representation types per context?

If not, propose to write documentation and / or where rule to prohibit.

Start with the linked buildingSMART forum discussion and the IFC product definition shape rules described in this issue. Determine whether multiple shape representations with different representation types may share one geometric context, then document the decision or specify the required where rule. Done means the intended constraint is resolved and recorded in the IFC4.x specification.

Written by the indexing model from the issue text.