Make it explict that starpu_mpi_task_build returns NULL on nodes which don't execute the task

doc/doxygen/chapters/16mpi_support.doxy

@@ -411,8 +411,8 @@ the cost of task submission.
 A function starpu_mpi_task_build() is also provided with the aim to
 only construct the task structure. All MPI nodes need to call the
 function, only the node which is to execute the task will return a
-valid task structure. Following the execution of the task, all nodes
-need to call the function starpu_mpi_task_post_build() -- with the same
+valid task structure, others will return NULL. That node must submit that task.
+All nodes then need to call the function starpu_mpi_task_post_build() -- with the same
 list of arguments as starpu_mpi_task_build() -- to post all the
 necessary data communications.
 

doc/doxygen/chapters/api/mpi.doxy

@@ -347,9 +347,9 @@ Create a task corresponding to codelet with the following arguments.
 The argument list must be zero-terminated. The function performs the
 first two steps of the function starpu_mpi_task_insert(). Only the MPI
 node selected in the first step of the algorithm will return a valid
-task structure which can then be submitted. The function
-starpu_mpi_task_post_build() MUST be called after the submission of
-the task, with the SAME list of arguments.
+task structure which can then be submitted, others will return NULL. The function
+starpu_mpi_task_post_build() MUST be called after that on all nodes, and after the submission of
+the task on the node which creates it, with the SAME list of arguments.
 
 \fn int starpu_mpi_task_post_build(MPI_Comm comm, struct starpu_codelet *codelet, ...)
 \ingroup API_MPI_Support