add note about the difference between children and slots

Author: chrisvfritz

src/guide/render-function.md

@@ -366,6 +366,21 @@ Vue.component('smart-list', {
 })
 ```
 
+### `slots` vs `children`
+
+You may wonder why we need both `slots` and `children`. Wouldn't `slots().default` be the same as `children`? In some cases, yes - but what if you have a functional component with the following children?
+
+``` html
+<my-functional-component>
+  <p slot="foo">
+    first
+  </p>
+  <p>second</p>
+</my-functional-component>
+```
+
+For this component, `children()` will give you both paragraphs, `slots().default` will give you only the second, and `slots().foo` will give you only the first. Having both `children` and `slots` therefore allows you to choose whether this component knows about a slot system or perhaps delegates that responsibility to another component by simply passing along `children`.
+
 ## Misc
 
 ### `keep-alive`