add comment explaining what the modules-split transformer does

lib/6to5/transformation/transformers/internal/modules-split.js

@@ -1,5 +1,11 @@
 "use strict";
 
+// in this transformer we have to split up classes and function declarations
+// from their exports. why? because sometimes we need to replace classes with
+// nodes that aren't allowed in the same contexts. also, if you're exporting
+// a generator function as a default then regenerator will destroy the export
+// declaration and leave a variable declaration in it's place... yeah, handy.
+
 var t = require("../../../types");
 
 exports.ExportDeclaration = function (node, parent, scope) {