docs: add error link when missing appDir: true (#43293)

[Slack
thread](https://vercel.slack.com/archives/C03KAR5DCKC/p1669206360940399)

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ]
[e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs)
tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

Co-authored-by: JJ Kasper <jj@jjsweb.site>
Co-authored-by: Jiachi Liu <inbox@huozhi.im>

errors/experimental-app-dir-config.md

@@ -0,0 +1,27 @@
+# Experimental `appDir: true`
+
+#### Why This Error Occurred
+
+Your project contains a directory named `app/`. Since Next.js 13, this is a [reserved name](https://nextjs.org/blog/next-13#new-app-directory-beta).
+
+#### Possible Ways to Fix It
+
+To use the new app directory and its features, please add `appDir: true` to your configuration in `next.config.js` under `experimental`.
+
+```js
+/** @type {import('next').NextConfig} */
+const nextConfig = {
+  experimental: {
+    appDir: true,
+  },
+}
+
+module.exports = nextConfig
+```
+
+If you do not want to use the new `app/` directory features yet, but have a directory named `app/`, rename the directory to something else.
+
+### Useful Links
+
+- [Next.js 13 App directory (Beta)](https://nextjs.org/blog/next-13#new-app-directory-beta)
+- [App directory (Beta) documentation](https://beta.nextjs.org/docs)

errors/manifest.json

@@ -761,6 +761,10 @@
         {
           "title": "next-router-not-mounted",
           "path": "/errors/next-router-not-mounted.md"
+        },
+        {
+          "title": "experimental-app-dir-config",
+          "path": "/errors/experimental-app-dir-config.md"
         }
       ]
     }

packages/next/lib/find-pages-dir.ts

@@ -41,12 +41,12 @@ export function findPagesDir(
   if (!isAppDirEnabled) {
     if (appDir != null && pagesDir == null) {
       throw new Error(
-        '> The `app` dir is experimental. Please add `{experimental:{appDir: true}}` to your `next.config.js` to enable it'
+        '> The `app` directory is experimental. To enable, add `appDir: true` to your `next.config.js` configuration under `experimental`. See https://nextjs.org/docs/messages/experimental-app-dir-config'
       )
     }
     if (appDir != null && pagesDir != null) {
       Log.warn(
-        'The `app` dir is experimental. Please add `{experimental:{appDir: true}}` to your `next.config.js` to enable it'
+        'The `app` directory is experimental. To enable, add `appDir: true` to your `next.config.js` configuration under `experimental`. See https://nextjs.org/docs/messages/experimental-app-dir-config'
       )
     }
     if (pagesDir == null) {

test/integration/appdir-missing-config/test/index.test.ts

@@ -14,7 +14,7 @@ const dir = path.join(__dirname, '..')
 const nextConfig = path.join(dir, 'next.config.js')
 const pagesIndex = path.join(dir, 'pages', 'index.js')
 const msg =
-  'The `app` dir is experimental. Please add `{experimental:{appDir: true}}` to your `next.config.js` to enable it'
+  'The `app` directory is experimental. To enable, add `appDir: true` to your `next.config.js` configuration under `experimental`. See https://nextjs.org/docs/messages/experimental-app-dir-config'
 
 function runTests(justPutIt: () => Promise<string>) {
   it('should print error when missing config with app', async () => {