Add ### Qt6 comment requested in code-review

Task-number: QTBUG-73484
Change-Id: I7c1c5faf3d32fa21d8d2277ec434084450290156
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/dbus/qdbusargument.h

@@ -158,6 +158,7 @@ QT_END_NAMESPACE
 Q_DECLARE_METATYPE(QDBusArgument)
 QT_BEGIN_NAMESPACE
 
+// ### Qt6: remove the defaulted T * = nullptr from these two (MSVC6 work-around):
 template<typename T> inline T qdbus_cast(const QDBusArgument &arg, T * = nullptr)
 {
     T item;