Add transparency technique docs for widgetitem

Fixes: QTBUG-132396
Pick-to: 6.9
Change-Id: Ibcbf5a0574a5d149777cf5d55b2a88d3c69d8540
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>

src/graphs3d/widget/q3dgraphswidgetitem.cpp

@@ -118,6 +118,35 @@ QList<QGraphsTheme *> Q3DGraphsWidgetItem::themes() const
     return d->m_graphsItem->themes();
 }
 
+/*!
+ * \property Q3DGraphsWidgetItem::transparencyTechnique
+ * \since 6.9
+ *
+ * \brief Specifies which transparency technique to use. The Default value is \c{Default}.
+ * When rendering transparent surface graphs, use \c{Approximate} or \c{Accurate}.
+ *
+ * \value Default
+ *        Indicates that order-independent transparency techniques are not used.
+ *        Offers the best performance. Use when graphs don't contain
+ *        transparency or when a bar or scatter graph is also using instancing,
+ *        that is \l optimizationHint is {QtGraphs3D::OptimizationHint::Default}.
+ *
+ * \value Approximate
+ *        Indicates that a graph attempts an approximation of order-independent
+ *        transparency. This method is faster than \c Accurate and works on older
+ *        hardware but may yield inaccurate results. Use when the order-independent
+ *        transparency is needed, but the performance cost has to be lower than
+ *        when using accurate order-independent transparency.
+ *
+ * \value Accurate
+ *        Indicates that accurate order-independent transparency is used.
+ *        Use when perfect transparency rendering is needed.
+ *        \note Accurate transparency is not yet implemented
+ *              and will be enabled when the required functionality
+ *              is added to QtQuick3D.
+ *
+ * \sa QtGraphs3D::TransparencyTechnique
+ */
 QtGraphs3D::TransparencyTechnique Q3DGraphsWidgetItem::transparencyTechnique() const
 {
     Q_D(const Q3DGraphsWidgetItem);