clarified lightning-cli help dev-setfees description

lightningd/chaintopology.c

@@ -656,7 +656,7 @@ static void json_dev_setfees(struct command *cmd,
 static const struct json_command dev_setfees_command = {
 	"dev-setfees",
 	json_dev_setfees,
-	"Set feerate in satoshi-per-kw for {immediate}, {normal} and {slow} (each optional) and show the value of those three feerates"
+	"Set feerate in satoshi-per-kw for {immediate}, {normal} and {slow} (each is optional, when set, separate by spaces) and show the value of those three feerates"
 };
 AUTODATA(json_command, &dev_setfees_command);