Improve documentation for --spread-job

Note in the salloc/sbatch/srun man pages that the --spread-job
  option disables the topology/tree plugin.

doc/man/man1/salloc.1

@@ -1267,9 +1267,10 @@ sockets.  See additional information under \fB\-B\fR option above when
 task/affinity plugin is enabled.
 
 .TP
-\fB\-\-spread\-job\fR>
+\fB\-\-spread\-job\fR
 Spread the job allocation over as many nodes as possible and attempt to
 evenly distribute tasks across the allocated nodes.
+This option disables the topology/tree plugin.
 
 .TP
 \fB\-\-switches\fR=<\fIcount\fR>[@<\fImax\-time\fR>]

doc/man/man1/sbatch.1

@@ -1407,9 +1407,10 @@ sockets.  See additional information under \fB\-B\fR option above when
 task/affinity plugin is enabled.
 
 .TP
-\fB\-\-spread\-job\fR>
+\fB\-\-spread\-job\fR
 Spread the job allocation over as many nodes as possible and attempt to
 evenly distribute tasks across the allocated nodes.
+This option disables the topology/tree plugin.
 
 .TP
 \fB\-\-switches\fR=<\fIcount\fR>[@<\fImax\-time\fR>]

doc/man/man1/srun.1

@@ -1879,6 +1879,7 @@ task/affinity plugin is enabled. This option applies to job allocations.
 \fB\-\-spread\-job\fR
 Spread the job allocation over as many nodes as possible and attempt to
 evenly distribute tasks across the allocated nodes.
+This option disables the topology/tree plugin.
 This option applies to job allocations.
 
 .TP