Document iteration order of multi-dict

[udalrich]

The iteration order when iterating over a MultiDict (e.g., with map or values is not specified anywhere in the documentation that I can find.

I suspect that the order is implementation dependent, which is fine. If that is the case, it should be documented.

In my current use case, I am hoping that the iteration order is the insertion order. Since I don't know the order, I don't know if I can use a MultiDict[A, B] or if I need to work with a Map[A, List[B]] which would be more cumbersome.