mention STARPU_OPENCL_ON_CPUS in socl documentation

doc/doxygen/chapters/20socl_opencl_extensions.doxy

@@ -70,4 +70,8 @@ Number of platforms:	2
 $
 \endverbatim
 
+To enable the use of CPU cores via OpenCL, one can set the STARPU_OPENCL_ON_CPUS
+environment variable to 1 and STARPU_NCPUS to 0 (to avoid using CPUs both via
+the OpenCL driver and the normal CPU driver).
+
 */