Add comment for udg_linear

tools/graphgen/graphGen_arg.ml

@@ -257,8 +257,16 @@ let (mkoptab : string array -> t -> unit) =
 
     mkopt args ["--progressive-prob";"-pp"]
       (Arg.Unit (fun () -> args.udg.proba <- LinearP))
-      [(["...";"...";"...\n"], "UDG")];
+      [(["Uses a probability function for UDG that change probabilty according to the distance from the center,";
+         "using a linear function. Lower the distance is, greater the probability is.";
+         "This probability function will be used to know if an edge will be created when";
+         "two Unit Discs touch each other.\n"
+        ], "UDG")];
 
+    (** [linear_proba r] gives a function that, for an input d (distance) outputs d/r. 
+    If r is the Unit Disc radius and d the distance between two points, 
+    it outputs a probability that is higher for a low d and lower for a high d. *)
+    
     mkopt args ["--dot_udg";"-du"]~arg:" <file-base-name>"
       (Arg.String (fun f -> match args.action with
                             | "UDG" -> args.dotUDG <- f