fixed docs on ReactElement that were out of sync with code

src/isomorphic/classic/element/ReactElement.js

@@ -30,8 +30,10 @@ var RESERVED_PROPS = {
 };
 
 /**
- * Base constructor for all React elements. This is only used to make this
- * work with a dynamic instanceof check. Nothing should live on this prototype.
+ * Factory method to create a new React element. This no longer adheres to
+ * the class pattern, so do not use new to call it. Also, no instanceof check
+ * will work. Instead test $$typeof field against Symbol.for('react.element') to check
+ * if something is a React Element.
  *
  * @param {*} type
  * @param {*} key