Document that the reduction codelet has to expose the STARPU_COMMUTE

ChangeLog

@@ -65,6 +65,10 @@ Small features:
   * Deprecate starpu_free() and add new function starpu_free_noflag()
     to specify allocated size.
 
+Changes:
+  * Require redux codelet to expose the STARPU_COMMUTE flag, since StarPU
+    actually uses commutability.
+
 Small changes:
   * Add a synthetic energy efficiency testcase.
   * Make reduction methods want the commute flag.