I don't think we want to say more than it says. If you pass a sequence or sequential collection, then it returns a "sequence of the elements" - it seems natural to expect the sequential elements to be returned in the same order.
On the flip side, if the coll passed to
distinct is unordered and not sequential (ie a map or set), this function cannot make promises about the order of the results.
It is also intentionally ambiguous which equal element you will get in the result (but again, the first in a sequential coll seems like the obvious answer).