Clarify whether one could use instance and instance container together

[angelo-v]

The registration entries map a type to a location and MUST use one of two predicates:

solid:instance - [...]
solid:instanceContainer - [...]

Can it use both or is it XOR?

It should read either "at least one of" or "exactly one of" to clarify this.

[jeff-zucker]

What would be the benefit of restricting it to one or the other? I can't think of one but I can see a case where some data was an instance and some a container. I can also see a case where different apps would have different entry-ways to the data - one using containers, another an index.

[angelo-v]

I am fine with having both, but would like to clarify the spec on that regard to say "MUST use at least one of two predicates"