async component

omitted the error retry feature because it is somewhat
useless when combined with native dynamic import caching behavior

Author: yyx990803

src/guide/components/async.md

@@ -1,5 +1,7 @@
 # Async Components
 
+## Basic Usage
+
 In large applications, we may need to divide the app into smaller chunks and only load a component from the server when it's needed. To make that possible, Vue has a [`defineAsyncComponent`](/api/general.html#defineasynccomponent) method:
 
 ```js
@@ -47,9 +49,31 @@ export default {
 
 </div>
 
-## Loading State
+## Loading and Error States
+
+Asynchronous operations inevitably involve loading and error states - `defineAsyncComponent()` supports handling these states via advanced options:
+
+```js
+const AsyncComp = defineAsyncComponent({
+  // the loader function
+  loader: () => import('./Foo.vue'),
+
+  // A component to use while the async component is loading
+  loadingComponent: LoadingComponent,
+  // Delay before showing the loading component. Default: 200ms.
+  delay: 200,
+
+  // A component to use if the load fails
+  errorComponent: ErrorComponent,
+  // The error component will be displayed if a timeout is
+  // provided and exceeded. Default: Infinity.
+  timeout: 3000
+})
+```
+
+If a loading component is provided, it will be displayed first while the inner component is being loaded. There is a default 200ms delay before the loading component is shown - this is because on fast networks, an instant loading state may get replaced too fast and end up looking like a flicker.
 
-## Error Handling
+If an error component is provided, it will be displayed when the Promise returned by the loader function is rejected. You can also specify a timeout to show the error component when the request is taking too long.
 
 ## Using with Suspense