title: Complexity of std::unordered_multiset insert
body: <p>Why is the worst case complexity of <code>std::unordered_multiset</code> insert is linear? I understand why is that the case for <code>std::unordered_set</code> (you have to check that inserted value is not in the set) but for multiset I don't get it. Am I missing something obvious?</p>