Document conformance issue about stencil attribute

Change-Id: I0008dffd4a919a17d2b6c3558790e197b55e7e4a
Task-number: QTBUG-45372
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>

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

@@ -200,5 +200,7 @@
  *
  *      \li The drawArrays and drawElements calls don't check for the validity of the enabled
  *      attributes or the buffers they point to.
+ *
+ *      \li Enabling the stencil context attribute also forces the depth attribute to be enabled.
  * \endlist
  */