It _does_ make clear that multiple UPDATEs to the same row are not allowed, but that in itself doesn't automatically restrict the use of multiple constraint targets; I could easily INSERT a set of values that would trigger that failure with just one constraint target.
http://www.postgresql.org/docs/devel/static/sql-insert.html talks about how MySQL's ON DUPLICATE can only act against the first matching row where multiple constraints match against multiple rows. I suppose if that were the case here (ie the first excluding row would stop other rows firing against the UPDATE) would break the deterministic feature, but it's not clear if that's true or not. I don't see why multiple target rows couldn't be updated based on multiple constraints, that would not in-and-of-itself break determinism.
