document that it is the MPI id that is used in the trace file

doc/doxygen/chapters/380_offline_performance_tools.doxy

@@ -84,7 +84,7 @@ Or you can simply point the <c>PKG_CONFIG_PATH</c> to
 When FxT is enabled, a trace is generated when StarPU is terminated by calling
 starpu_shutdown(). The trace is a binary file whose name has the form
 <c>prof_file_XXX_YYY</c> where <c>XXX</c> is the user name, and
-<c>YYY</c> is the pid of the process that used StarPU. One can change
+<c>YYY</c> is the MPI id of the process that used StarPU. One can change
 the name of the file by setting the environnement variable \ref
 STARPU_FXT_SUFFIX. This file is saved in the
 <c>/tmp/</c> directory by default, or by the directory specified by