improve render function section at guide (#473)

* improve render function section at guide

* Small rewording of directives example

Author: kazupon

src/guide/render-function.md

@@ -160,6 +160,20 @@ One thing to note: similar to how `v-bind:class` and `v-bind:style` have special
   nativeOn: {
     click: this.nativeClickHandler
   },
+  // Custom directives. Note that the binding's 
+  // oldValue cannot be set, as Vue keeps track
+  // of it for you.
+  directives: [
+    {
+      name: 'my-custom-directive', 
+      value: '2'
+      expression: '1 + 1',
+      arg: 'foo',
+      modifiers: {
+        bar: true
+      }
+    }
+  ],
   // The name of a slot if the child of a component
   slot: 'name-of-slot'
   // Other special top-level properties