doc/chapters/mpi-support.texi: add todo

doc/chapters/mpi-support.texi

@@ -309,6 +309,7 @@ The algorithm also includes a communication cache mechanism that
 allows not to send data twice to the same MPI node, unless the data
 has been modified. The cache can be disabled
 (@pxref{STARPU_MPI_CACHE}).
+@c todo parler plus du cache
 
 @end deftypefun