Threading Types: add a bit of documentation to subnet type.

This explains how the length field is currently handled when exchanging
data in both directions.

src/threading/SerialTypes.h

@@ -106,6 +106,13 @@ struct Value {
 		} in;
 	};
 
+	// A small note for handling subnet values: Subnet values emitted from
+	// the logging framework will always have a length that is based on the
+	// internal IPv6 representation (so you have to substract 96 from it to
+	// get the correct value for IPv4).
+	// However, the Input framework expects the "normal" length for an IPv4
+	// address (so do not add 96 to it), because the underlying constructors
+	// for the SubNet type want it like this.
 	struct subnet_t { addr_t prefix; uint8_t length; };
 
 	/**