expand explanation of ControlMachine configuration parameter

doc/man/man5/slurm.conf.5

-.TH "slurm.conf" "5" "April 2008" "slurm.conf 1.3" "Slurm configuration file"
+.TH "slurm.conf" "5" "May 2008" "slurm.conf 1.3" "Slurm configuration file"
 .SH "NAME"
 slurm.conf \- Slurm configuration file 
 .SH "DESCRIPTION"
@@ -187,8 +187,9 @@ By default the \fBControlAddr\fR will be identical in value to
 
 .TP
 \fBControlMachine\fR
-The name of the machine where SLURM control functions are executed. 
-This should be a node name without the full domain name (e.g. "lx0001"). 
+The name of the machine where SLURM control functions are executed
+as returned by the \fIgethostname()\fR function the cut at the first dot
+or the \fIhostname \-s\fR command (e.g. use "tux001" rather than "tux001.my.com"). 
 This value must be specified.
 See  the \fBRELOCATING CONTROLLERS\fR section if you change this.