clarify createElement arguments, fixes vuejs#1524

Author: chrisvfritz

src/v2/guide/render-function.md

@@ -138,8 +138,8 @@ The next thing you'll have to become familiar with is how to use template featur
 // @returns {VNode}
 createElement(
   // {String | Object | Function}
-  // An HTML tag name, component options, or function
-  // returning one of these. Required.
+  // An HTML tag name, component options, or async
+  // function resolving to one of these. Required.
   'div',
 
   // {Object}
@@ -605,7 +605,7 @@ If you are using template-based functional components, you will also have to man
 <template functional>
   <button
     class="btn btn-primary"
-    v-bind="data.attrs" 
+    v-bind="data.attrs"
     v-on="listeners"
   >
     <slot/>