clarify that mpich1_shmem only launches one task per node

doc/html/mpi_guide.shtml

@@ -265,6 +265,12 @@ with the <i>--mpi=mpich1_shmem</i> option.</p>
 $ srun -n16 --mpi=mpich1_shmem a.out
 </pre>
 
+<p>NOTE: Using a configuration of <i>MpiDefault=mpich1_shmem</i> will result in
+one task being launched per node with the expectation that the MPI library will
+launch the remaining tasks based upon environment variables set by SLURM.
+Non-MPI jobs started in this configuration will lack the mechanism to launch
+more than one task per node unless srun's <i>--mpi=none</i> option is used.</p>
+
 <p>If you are using MPICH P4 (<i>DEFAULT_DEVICE=ch_p4</i> in
 the mpirun script) and SLURM version 1.2.11 or newer,
 then it is recommended that you apply the patch in the SLURM

doc/man/man5/slurm.conf.5

@@ -764,10 +764,16 @@ May not exceed 65533.
 Identifies the default type of MPI to be used.
 Srun may override this configuration parameter in any case.
 Currently supported versions include:
+\fBlam\fR,
+\fBmpich1_p4\fR,
+\fBmpich1_shmem\fR,
 \fBmpichgm\fR,
+\fBmpichmx\fR,
 \fBmvapich\fR,
-\fBnone\fR (default, which works for many other versions of MPI including
-LAM MPI and Open MPI).
+\fBnone\fR (default, which works for many other versions of MPI) and
+\fBopenmpi\fR.
+More information about MPI use is available here
+<https://computing.llnl.gov/linux/slurm/mpi_guide.html>.
 
 .TP
 \fBMpiParams\fR