add documentation for STARPU_USE_NUMA

doc/doxygen/chapters/501_environment_variables.doxy

@@ -1107,6 +1107,19 @@ implements an advanced but centralized management of concurrent data
 accesses (see \ref ConcurrentDataAccess).
 </dd>
 
+<dt>STARPU_USE_NUMA</dt>
+<dd>
+\anchor STARPU_USE_NUMA 
+\addindex __env__STARPU_USE_NUMA
+When defined, NUMA nodes are taking into account by StarPU. Otherwise, memory
+is considered as only one node. This is experimental for now.
+
+When enabled, STARPU_MAIN_MEMORY is a pointer to the NUMA node associated to the
+first CPU worker if it exists, the NUMA node associated to the first GPU discovered otherwise.
+If StarPU doesn't find any NUMA node after these step, STARPU_MAIN_MEMORY is the first NUMA node
+discovered by StarPU.
+</dd>
+
 </dl>
 
 \section ConfiguringTheHypervisor Configuring The Hypervisor