fixes a 'missing argument' case for 'scontrol --clusters'.

Based on 03_scontrol_clusters_argu~1.diff from Gerrit.

doc/man/man1/scontrol.1

@@ -29,7 +29,7 @@ and jobs steps. This causes information to be displayed about partitions
 that are configured as hidden and partitions that are unavailable to user's
 group.
 .TP
-\fB\-d\fR, \fB\-\-detail\fR
+\fB\-d\fR, \fB\-\-details\fR
 Causes the \fBshow\fR command to provide additional details where available.
 .TP
 \fB\-h\fR, \fB\-\-help\fR
@@ -142,7 +142,7 @@ The two \fISPECIFICATION\fP choices are \fIPartitionName=<name>\fP and
 \fIBlockName=<name>\fP also works.
 
 .TP
-\fBdetail\fP
+\fBdetails\fP
 Causes the \fIshow\fP command to provide additional details where available,
 namely the specific CPUs and NUMA memory allocated on each node.
 Note that on computers with hyperthreading enabled and SLURM configured to
@@ -151,7 +151,7 @@ Each hyperthread on that core can be allocated a separate task, so a job's
 CPU count and task count may differ.
 See the \fB\-\-cpu_bind\fR and \fB\-\-mem_bind\fR option descriptions in
 srun man pages for more information.
-The \fBdetail\fP option is currently only supported for the \fIshow job\fP
+The \fBdetails\fP option is currently only supported for the \fIshow job\fP
 command.
 
 .TP

src/scontrol/scontrol.c

@@ -91,8 +91,8 @@ main (int argc, char *argv[])
 	int option_index;
 	static struct option long_options[] = {
 		{"all",      0, 0, 'a'},
-		{"cluster",  0, 0, 'M'},
-		{"clusters", 0, 0, 'M'},
+		{"cluster",  1, 0, 'M'},
+		{"clusters", 1, 0, 'M'},
 		{"details",  0, 0, 'd'},
 		{"help",     0, 0, 'h'},
 		{"hide",     0, 0, OPT_LONG_HIDE},
@@ -1514,7 +1514,7 @@ _usage () {
 scontrol [<OPTION>] [<COMMAND>]                                            \n\
     Valid <OPTION> values are:                                             \n\
      -a or --all: equivalent to \"all\" command                            \n\
-     -d or --detail: equivalent to \"detail\" command                      \n\
+     -d or --details: equivalent to \"details\" command                    \n\
      -h or --help: equivalent to \"help\" command                          \n\
      --hide: equivalent to \"hide\" command                                \n\
      -M or --cluster: equivalent to \"cluster\" command                    \n\
@@ -1540,7 +1540,7 @@ scontrol [<OPTION>] [<COMMAND>]                                            \n\
      completing               display jobs in completing state along with  \n\
                               their completing or down nodes               \n\
      create <SPECIFICATIONS>  create a new partition or reservation        \n\
-     detail                   evokes additional details from the \"show\"  \n\
+     details                  evokes additional details from the \"show\"  \n\
                               command                                      \n\
      delete <SPECIFICATIONS>  delete the specified partition or reservation\n\
                               On Dynamic layout Bluegene systems you can also\n\