Clarify rule for setting facet types

[bobylito]

Currently, a facet can be set to multiple type. But it doesn't work properly due to the implementation of disjunctive and hierarchical facets.

Proposition: enforce a single type, and document it.

[vvo]

Do you mean by enforce => Throw?

[bobylito]

Yes I would say. But there is gradiant of options like not letting the user do it but do not break anything (that's a silly one I guess)