Updates TIMER_UNIQUE documentation (#34328)

code/__DEFINES/subsystems.dm

@@ -5,6 +5,7 @@
 
 //Timing subsystem
 //Don't run if there is an identical unique timer active
+//if the arguments to addtimer are the same as an existing timer, it doesn't create a new timer, and returns the id of the existing timer
 #define TIMER_UNIQUE		0x1
 //For unique timers: Replace the old timer rather then not start this one
 #define TIMER_OVERRIDE		0x2
@@ -141,4 +142,4 @@
 			A.overlays.Cut();\
 		}\
 		A.flags_1 &= ~OVERLAY_QUEUED_1;\
-	}
+	}