Add documentation to boot.js

This file's jsdoc had diverged a good bit from the internal version

lib/utils/boot.js

@@ -8,6 +8,18 @@ Code distributed by Google as part of the polymer project is also
 subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 */
 
-window.JSCompiler_renameProperty = function(prop) { return prop; };
+/* eslint-disable no-unused-vars */
+/**
+ * When using Closure Compiler, JSCompiler_renameProperty(property, object) is replaced by the munged name for object[property]
+ * We cannot alias this function, so we have to use a small shim that has the same behavior when not compiling.
+ *
+ * @param {string} prop
+ * @param {?Object} obj
+ * @return {string}
+ */
+window.JSCompiler_renameProperty = function(prop, obj) {
+  return prop;
+};
+/* eslint-enable */
 
 export {};