Add docs for static 404 and pages/404

docs/advanced-features/custom-error-page.md

@@ -2,9 +2,34 @@
 description: Override and extend the built-in Error page to handle custom errors.
 ---
 
-# Custom Error Page
+## 404 Page
 
-**404** or **500** errors are handled both client-side and server-side by the `Error` component. If you wish to override it, define the file `pages/_error.js` and add the following code:
+A 404 page may be accessed very often. Rendering the `_error` page for every visit increases the load for your server or the invocations for serverless functions. This can result in increased costs and slow experiences.
+
+To avoid the above pitfalls, Next.js provides a static 404 page by default without having to add any additional files.
+
+The static 404 page is automatically statically optimized at build time and used with `next start` and when deploying on [ZEIT Now](https://zeit.co).
+
+If you have a custom `pages/_app.js` with `getInitialProps`, the automatic static optimization will not be able to be applied since you are defining a global data dependency.
+
+### Customizing The 404 Page
+
+To create a custom 404 page you can create a `pages/404.js` file. This file is meant to allow customizing the 404 page while keeping the page static and doesn't allow using any data-fetching methods like `getStaticProps`, `getServerSideProps`, or `getInitialProps`.
+
+```jsx
+// pages/404.js
+export default function custom404() {
+  return <h1>404 - Page Not Found</h1>
+}
+```
+
+## 500 Page
+
+By default Next.js provides an error page that matches the default 404 page's style. This page is not statically optimized like the 404 page and is why we allow creating a separate `pages/404.js` file to enable you to report 500 errors without de-optimizing your 404 page.
+
+### Customizing The Error Page
+
+**500** errors are handled both client-side and server-side by the `Error` component. If you wish to override it, define the file `pages/_error.js` and add the following code:
 
 ```jsx
 function Error({ statusCode }) {
@@ -27,7 +52,7 @@ export default Error
 
 > `pages/_error.js` is only used in production. In development you'll get an error with the call stack to know where the error originated from.
 
-## Reusing the built-in error page
+### Reusing the built-in error page
 
 If you want to render the built-in error page you can by importing the `Error` component: