Decide and document how inOrder() works with Multimap

Currently, it checks that the `keySet` is in the expected order and that the values for each key are in the expected order.

This is different than checking whether all entries are in the expected order, at least in the strange case of `LinkedListMultimap` (often used for URL query parameters).

This distinction is probably not important to most users. Still, we should decide which way we want to do it, and we should document accordingly.