Make comment more informative

GPIO/iCE40/ring.v

@@ -2,7 +2,7 @@
 `default_nettype none
 
 module top (
-    input CLK_12MHZ,           // system clock
+    input CLK_12MHZ, // the iCEBreaker has an external 12MHz oscillator
     output P1A1,
     input P1A2,
 );