Document QHash element reference invalidation on rehash

#include <QtCore/QCoreApplication>
#include <QtCore/QHash>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QHash<QObject*, QList<int>> hash;

    constexpr size_t kQObjectCount = 1000;

    // I want to insert items into a reference to a list inside the QHash
    // at the same time as adding elements to the QHash.
    QObject* currentObject{nullptr};

    for (size_t i; i < 64; ++i) {
        currentObject = new QObject(&a);
        hash.insert(currentObject, {});
    }

    // Take a reference to a value in the hash.
    QList<int>& list = hash[currentObject];
    list.append(1);
    list.append(3);
    list.append(2);
    Q_ASSERT(hash.capacity() == 64);

    Q_ASSERT(list.contains(1));
    Q_ASSERT(list.contains(2));
    Q_ASSERT(list.contains(3));

    // Add one more object to the hash to cause a rehash.
    hash.insert(new QObject(&a), {});
    Q_ASSERT(hash.capacity() != 64);

    list.append(4);
    list.append(5);
    list.append(6);

    // I would expect the list to now contain 1, 2, 3, 4, 5 & 6
    // But it doesn't as the reference has been invalidated.
    Q_ASSERT(list.contains(1)); // <-- This fails.
    Q_ASSERT(list.contains(2));
    Q_ASSERT(list.contains(3));
    Q_ASSERT(list.contains(4));
    Q_ASSERT(list.contains(5));
    Q_ASSERT(list.contains(6));

    return 0;
}