Add instanceof memory leak warning to _.isArray.

Former-commit-id: f90b1ad1850fb21c1d976f037a382c7388496d1c

lodash.js

@@ -1180,6 +1180,8 @@
    * // => true
    */
   var isArray = nativeIsArray || function(value) {
+    // `instanceof` may cause a memory leak in IE 7 if `value` is a host object
+    // http://ajaxian.com/archives/working-aroung-the-instanceof-memory-leak
     return value instanceof Array || toString.call(value) == arrayClass;
   };