Update trigger man page

A configuration change trigger event occurs when a node state
changes (e.g. Up, Down, Drain, etc.)

doc/man/man1/strigger.1

@@ -208,6 +208,8 @@ This can be useful to clear triggers which may have already been purged.
 .TP
 \fB\-r\fR, \fB\-\-reconfig\fR
 Trigger an event when the system configuration changes.
+This is triggered when the slurmctld daemon reads its configuration file or
+when a node state changes.
 
 .TP
 \fB\-\-set\fP