Added conformance comment about alpha

At least on windows, trying to create a context with 0 alpha bits and
8 bits for other colors creates a context with 8 alpha bits.

Change-Id: I23659e4d47a0adee38249d85034a2bac41b84d1f
Task-number: QTBUG-45588
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>

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

@@ -202,5 +202,7 @@
  *      attributes or the buffers they point to.
  *
  *      \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.
  * \endlist
  */