add comment for obscure snippet

mithril.js

@@ -91,6 +91,7 @@ var m = (function app(window, undefined) {
 		//there's logic that relies on the assumption that null and undefined data are equivalent to empty strings
 		//- this prevents lifecycle surprises from procedural helpers that mix implicit and explicit return statements (e.g. function foo() {if (cond) return m("div")}
 		//- it simplifies diffing code
+		//data.toString() is null if data is the return value of Console.log in Firefox
 		if (data == null || data.toString() == null) data = "";
 		if (data.subtree === "retain") return cached;
 		var cachedType = type.call(cached), dataType = type.call(data);