docs: return values for render functions

vuejs/docs@da874b2#diff-0b635f32793fe88d869035c17f5143d6a7011183741021e19963a52beb70927a

Author: naokie

src/guide/render-function.md

@@ -567,6 +567,33 @@ render () {
 }
 ```
 
+## Return Values for Render Functions
+
+In all of the examples we've seen so far, the `render` function has returned a single root VNode. However, there are alternatives.
+
+Returning a string will create a text VNode, without any wrapping element:
+
+```js
+render() {
+  return 'Hello world!'
+}
+```
+
+We can also return an array of children, without wrapping them in a root node. This creates a fragment:
+
+```js
+// Equivalent to a template of `Hello<br>world!`
+render() {
+  return [
+    'Hello',
+    h('br'),
+    'world!'
+  ]
+}
+```
+
+If a component needs to render nothing, perhaps because data is still loading, it can just return `null`. This will be rendered as a comment node in the DOM.
+
 ## JSX
 
 たくさんの `render` 関数を書いていると、こういう感じのものを書くのがつらく感じるかもしれません: