QQuickItem doc fix

Note to self, I'll contrib the fix.

https://doc-snapshots.qt.io/qt5-5.9/qquickitem.html#details

From:

"All visual QML items are rendered using the scene graph, a low-level, high-performance rendering stack, closely tied to OpenGL."

To:

"All visual QML items are rendered using the scene graph, the default implementation of which is a low-level, high-performance rendering stack, closely tied to OpenGL."