Documented some conformance breaks

- possible OUT_OF_MEMORY with canvas of MAX_VIEWPORT_DIMS
- extra methods for contexts

Change-Id: I807287692e200e08abac242d611bdce01a7136a5
Task-number: QTBUG-45728
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>

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

@@ -204,5 +204,11 @@
  *      \li Enabling the stencil context attribute also forces the depth attribute to be enabled.
  *
  *      \li Disabling alpha context attribute does not work on some platforms.
+ *
+ *      \li Trying to set canvas size to MAX_VIEWPORT_DIMS may result in OUT_OF_MEMORY on some
+ *      graphics cards.
+ *
+ *      \li The following extra methods exist for contexts: objectNameChanged, nameChanged,
+ *      canvasChanged, drawingBufferWidthChanged, and drawingBufferHeightChanged.
  * \endlist
  */