Specify that `type[]` distributes over unions

[InSyncWithFoo]

I think this much can be inferred, since the specs already said that:

• type[A | B] is supported, and
• B the class is a valid value for type[A | B]

[erictraut]

Since this is a substantive change to the spec, it should go through formal TC review. Please follow the process documented here.

[InSyncWithFoo]

Backlink to discussion: Specs clarification: type[A | B] is equivalent/the same as type[A] | type[B].