mpi: document STARPU_MPI_COMM

ChangeLog

@@ -49,6 +49,8 @@ New features:
 	- New selection node policies can be un/registered with the
 	  functions starpu_mpi_node_selection_register_policy() and
 	  starpu_mpi_node_selection_unregister_policy()
+	- New environment variable STARPU_MPI_COMM which enables
+	  basic tracing of communications.
 
   * New STARPU_COMMUTE flag which can be passed along STARPU_W or STARPU_RW to
     let starpu commute write accesses.

doc/doxygen/chapters/40environment_variables.doxy

@@ -1,7 +1,7 @@
 /*
  * This file is part of the StarPU Handbook.
  * Copyright (C) 2009--2011  Universit@'e de Bordeaux
- * Copyright (C) 2010, 2011, 2012, 2013, 2014  Centre National de la Recherche Scientifique
+ * Copyright (C) 2010, 2011, 2012, 2013, 2014, 2015  Centre National de la Recherche Scientifique
  * Copyright (C) 2011, 2012 Institut National de Recherche en Informatique et Automatique
  * See the file version.doxy for copying conditions.
  */
@@ -447,6 +447,16 @@ to 0. It is enabled by default or for any other values of the variable
 \ref STARPU_MPI_CACHE.
 </dd>
 
+<dt>STARPU_MPI_COMM</dt>
+<dd>
+\anchor STARPU_MPI_COMM
+\addindex __env__STARPU_MPI_COMM
+Communication trace for starpumpi (\ref MPISupport) will be
+enabled when the environment variable \ref STARPU_MPI_COMM is set
+to 1, and StarPU has been configured with the option
+\ref enable-verbose "--enable-verbose".
+</dd>
+
 <dt>STARPU_MPI_CACHE_STATS</dt>
 <dd>
 \anchor STARPU_MPI_CACHE_STATS