Proposed resolution:

Add explicit.

namespace std {
template <class Key,
class Hash = hash<Key>,
class Pred = std::equal_to<Key>,
class Alloc = std::allocator<Key> >
class unordered_multiset
{
public:
...
explicit unordered_multiset(const Allocator&);

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Addresses JP-13

Constructor accepting an allocator as a single parameter should be qualified as explicit.