add rest parameter comments

lib/6to5/transformation/transformers/es6-rest-parameters.js

@@ -10,6 +10,8 @@ exports.Function = function (node, parent, file) {
   t.ensureBlock(node);
 
   var argsId = t.identifier("arguments");
+
+  // otherwise `arguments` will be remapped in arrow functions
   argsId._ignoreAliasFunctions = true;
 
   var start = t.literal(node.params.length);
@@ -17,6 +19,9 @@ exports.Function = function (node, parent, file) {
 
   var arrKey = key;
   if (node.params.length) {
+    // this method has additional params, so we need to subtract
+    // the index of the current argument position from the
+    // position in the array that we want to populate
     arrKey = t.binaryExpression("-", arrKey, start);
   }