doc: reorganise mpi sections and mention starpu_data_set_tag needs to be called

doc/chapters/api.texi

@@ -3188,6 +3188,7 @@ start recording it again, etc.
 * Communication::
 * Communication Cache::
 * MPI Insert Task::
+* Collective Operations::
 @end menu
 
 @node Initialisation
@@ -3419,6 +3420,29 @@ owner if needed. At least the target node and the owner have to call the
 function.
 @end deftypefun
 
+@node Collective Operations
+@subsection Collective Operations
+
+@deftypefun int starpu_mpi_scatter_detached (starpu_data_handle_t *@var{data_handles}, int @var{count}, int @var{root}, MPI_Comm @var{comm}, {void (*}@var{scallback})(void *), {void *}@var{sarg}, {void (*}@var{rcallback})(void *), {void *}@var{rarg})
+Scatter data among processes of the communicator based on the ownership of
+the data. For each data of the array @var{data_handles}, the
+process @var{root} sends the data to the process owning this data.
+Processes receiving data must have valid data handles to receive them.
+On completion of the collective communication, the @var{scallback} function is
+called with the argument @var{sarg} on the process @var{root}, the @var{rcallback} function is
+called with the argument @var{rarg} on any other process.
+@end deftypefun
+
+@deftypefun int starpu_mpi_gather_detached (starpu_data_handle_t *@var{data_handles}, int @var{count}, int @var{root}, MPI_Comm @var{comm}, {void (*}@var{scallback})(void *), {void *}@var{sarg}, {void (*}@var{rcallback})(void *), {void *}@var{rarg})
+Gather data from the different processes of the communicator onto the
+process @var{root}. Each process owning data handle in the array
+@var{data_handles} will send them to the process @var{root}. The
+process @var{root} must have valid data handles to receive the data.
+On completion of the collective communication, the @var{rcallback} function is
+called with the argument @var{rarg} on the process @var{root}, the @var{scallback} function is
+called with the argument @var{sarg} on any other process.
+@end deftypefun
+
 @node Task Bundles
 @section Task Bundles
 

doc/chapters/mpi-support.texi

@@ -335,27 +335,8 @@ execute them, or to send the required data).
 @node MPI Collective Operations
 @section MPI Collective Operations
 
-@deftypefun int starpu_mpi_scatter_detached (starpu_data_handle_t *@var{data_handles}, int @var{count}, int @var{root}, MPI_Comm @var{comm}, {void (*}@var{scallback})(void *), {void *}@var{sarg}, {void (*}@var{rcallback})(void *), {void *}@var{rarg})
-Scatter data among processes of the communicator based on the ownership of
-the data. For each data of the array @var{data_handles}, the
-process @var{root} sends the data to the process owning this data.
-Processes receiving data must have valid data handles to receive them.
-On completion of the collective communication, the @var{scallback} function is
-called with the argument @var{sarg} on the process @var{root}, the @var{rcallback} function is
-called with the argument @var{rarg} on any other process.
-@end deftypefun
-
-@deftypefun int starpu_mpi_gather_detached (starpu_data_handle_t *@var{data_handles}, int @var{count}, int @var{root}, MPI_Comm @var{comm}, {void (*}@var{scallback})(void *), {void *}@var{sarg}, {void (*}@var{rcallback})(void *), {void *}@var{rarg})
-Gather data from the different processes of the communicator onto the
-process @var{root}. Each process owning data handle in the array
-@var{data_handles} will send them to the process @var{root}. The
-process @var{root} must have valid data handles to receive the data.
-On completion of the collective communication, the @var{rcallback} function is
-called with the argument @var{rarg} on the process @var{root}, the @var{scallback} function is
-called with the argument @var{sarg} on any other process.
-@end deftypefun
-
-@page
+The functions are described in @ref{Collective Operations}.
+
 @cartouche
 @smallexample
 if (rank == root)
@@ -388,6 +369,7 @@ for(x = 0; x < nblocks ;  x++)
     @}
     if (data_handles[x]) @{
         starpu_data_set_rank(data_handles[x], mpi_rank);
+        starpu_data_set_tag(data_handles[x], x*nblocks+y);
     @}
 @}