document-node(a|b) is the same type as document-node(a)|document-node(b)

[michaelhkay]

document-node(a|b) is the same type as document-node(a) | document-node(b) but the current subtyping rules don't say this.

Revealed by test case misc/subtyping-076

Note, this problem existed before we introduced document-node(X), the same is true of the expansion:

document-node(element(a|b)) is the same type as document-node(element(a)) | document-node(element(b))