QMultiHashIterator is not documented

Since QMultiHash no longer derives from QHash in Qt6, QMultiHashIterator was indroduced but it has no documentation.
Even the QMultiHash documentation is pointing to QHashIterator ("and you can traverse a QMultiHash using QHash's iterator classes (for example, QHashIterator).") which is wrong.

Initial report from the forum: https://forum.qt.io/topic/152336/qhashiterator-with-qmultihash