Added conformance note about buffer checks in draw calls

Task-number: QTBUG-45074
Task-number: QTBUG-45195
Change-Id: I00a9f300304c426aa49942af5111f1fc681c91c6
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>

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

@@ -197,5 +197,8 @@
  *
  *      \li No special error detection for binding attributes to overlapping locations
  *      (i.e. attribute aliasing) when linking a shader program.
+ *
+ *      \li The drawArrays and drawElements calls don't check for the validity of the enabled
+ *      attributes or the buffers they point to.
  * \endlist
  */