Document QML_CONSTRUCTIBLE_VALUE and QML_STRUCTURED_VALUE

From ulherman's comment at QTBUG-56611: "You can register named value types with the QML_VALUE_TYPE macro these days. There is also QML_CONSTRUCTIBLE_VALUE and QML_STRUCTURED_VALUE. With those you can initialize value type properties from either their constructor parameters or from a JavaScript object containing their properties."

Only QML_VALUE_TYPE is documented. The other 2 are not:

• https://doc.qt.io/qt-6/qqmlengine.html#QML_VALUE_TYPE
• https://doc.qt.io/qt-6/whatsnew64.html#qt-qml-module