Document safety expectations, or the lack thereof

I noticed that the instances for `Map`, `Set`, `IntMap`, and `IntSet` are "unsafe" in the sense that they will happily build structures that don't obey the expected invariants if they are presented with improperly ordered input. Is this acceptable behavior (i.e., is `get` only supposed to be used with trusted input)? Or should the invocations of `fromDistinctAscList` be replaced with `fromList`?