Document the non-conformance with toDataUrl and drawImage.

Change-Id: Icf174fbfcefc124ce4fa8985284b166a3e7eee23
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>

src/imports/qtcanvas3d/doc/src/qtcanvas3d.qdoc

@@ -226,5 +226,10 @@
  *
  *      \li The following extra methods exist for contexts: objectNameChanged, nameChanged,
  *      canvasChanged, drawingBufferWidthChanged, and drawingBufferHeightChanged.
+ *
+ *      \li Canvas3D doesn't have toDataUrl() method.
+ *
+ *      \li Canvas3D item can't be given as a parameter to
+ *      \l{Context2D::drawImage()}{Context2D.drawImage()}.
  * \endlist
  */