change anonymous functions to named in docs examples (#17510)

Fixes #17200

docs/advanced-features/preview-mode.md

@@ -39,7 +39,7 @@ First, create a **preview API route**. It can have any name - e.g. `pages/api/pr
 In this API route, you need to call `setPreviewData` on the response object. The argument for `setPreviewData` should be an object, and this can be used by `getStaticProps` (more on this later). For now, we’ll use `{}`.
 
 ```js
-export default (req, res) => {
+export default function handler(req, res) {
   // ...
   res.setPreviewData({})
   // ...
@@ -54,7 +54,7 @@ You can test this manually by creating an API route like below and accessing it
 // A simple example for testing it manually from your browser.
 // If this is located at pages/api/preview.js, then
 // open /api/preview from your browser.
-export default (req, res) => {
+export default function handler(req, res) {
   res.setPreviewData({})
   res.end('Preview mode enabled')
 }
@@ -175,7 +175,7 @@ By default, no expiration date is set for the preview mode cookies, so the previ
 To clear the preview cookies manually, you can create an API route which calls `clearPreviewData` and then access this API route.
 
 ```js
-export default (req, res) => {
+export default function handler(req, res) {
   // Clears the preview mode cookies.
   // This function accepts no arguments.
   res.clearPreviewData()

docs/api-routes/dynamic-api-routes.md

@@ -16,7 +16,7 @@ API routes support [dynamic routes](/docs/routing/dynamic-routes.md), and follow
 For example, the API route `pages/api/post/[pid].js` has the following code:
 
 ```js
-export default (req, res) => {
+export default function handler(req, res) {
   const {
     query: { pid },
   } = req
@@ -68,7 +68,7 @@ And in the case of `/api/post/a/b`, and any other matching path, new parameters
 An API route for `pages/api/post/[...slug].js` could look like this:
 
 ```js
-export default (req, res) => {
+export default function handler(req, res) {
   const {
     query: { slug },
   } = req

docs/api-routes/introduction.md

@@ -22,7 +22,7 @@ Any file inside the folder `pages/api` is mapped to `/api/*` and will be treated
 For example, the following API route `pages/api/user.js` handles a `json` response:
 
 ```js
-export default (req, res) => {
+export default function handler(req, res) {
   res.statusCode = 200
   res.setHeader('Content-Type', 'application/json')
   res.end(JSON.stringify({ name: 'John Doe' }))
@@ -37,7 +37,7 @@ For an API route to work, you need to export as default a function (a.k.a **requ
 To handle different HTTP methods in an API route, you can use `req.method` in your request handler, like so:
 
 ```js
-export default (req, res) => {
+export default function handler(req, res) {
   if (req.method === 'POST') {
     // Process a POST request
   } else {

docs/api-routes/response-helpers.md

@@ -15,7 +15,7 @@ description: API Routes include a set of Express.js-like methods for the respons
 The response (`res`) includes a set of Express.js-like methods to improve the developer experience and increase the speed of creating new API endpoints, take a look at the following example:
 
 ```js
-export default (req, res) => {
+export default function handler(req, res) {
   res.status(200).json({ name: 'Next.js' })
 }
 ```