QML ListModel should document that it's a QAIM

ListModel instantiates QQmlListModel, which inherits from QAbstractListModel. QQmlListModel is a private class, so we do not really want to mention it. However, we should still mention that it is based on the public QAbstractListModel, and ensure that there is a link to QALM.

Moreover, we should highlight that one can use invokable methods from QALM/QAIM like index to retrieve the QModelIndex, which is useful in conjunction with e.g. ItemSelectionModel