chore: update middleware docs wording (#799)

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Author: ascorbic

docs/middleware.md

@@ -1,11 +1,11 @@
 # Using Next 12 middleware on Netlify
 
-Next 12 introduces a new feature called [Middleware](https://nextjs.org/docs/middleware), in which functions
-run before a request has finished processing. Middleware can be used to modify the request or replace the response. For
-example, it can change headers, rewrite the request path, or return a different response entirely.
+Next 12 introduces a new feature called [Middleware](https://nextjs.org/docs/middleware), in which functions run before
+a request has finished processing. Middleware can be used to modify the request or replace the response. For example, it
+can change headers, rewrite the request path, or return a different response entirely.
 
-Next.js Middleware can run either in an edge function or at the origin. On Netlify, middleware runs at the origin as part
-of the normal Next.js server.
+Next.js Middleware can run either in an edge function or at the origin. On Netlify, middleware runs at the origin as
+part of the normal Next.js server.
 
 ## How to deploy Next 12 middleware
 
@@ -15,8 +15,8 @@ workarounds that are currently required for some features during the beta period
 
 ### `geo`
 
-When running at the origin, Next.js does not populate the `request.geo` object. Fortunately there is a one line fix to get
-the visitor's country:
+When running at the origin, Next.js does not populate the `request.geo` object. Fortunately there is a one line fix to
+get the visitor's country:
 
 ```typescript
 export async function middleware(req: NextRequest) {
@@ -42,35 +42,31 @@ export async function middleware(req: NextRequest) {
 
 ## Caveats
 
-Because the middleware runs at the origin, it is run _after_ Netlify rewrites and redirects. If a static file is served by the Netlify CDN 
-then the middleware is never run, as middleware only runs when a page is served by Next.js. This means that middleware should not be used with the
-`EXPERIMENTAL_MOVE_STATIC_FILES` option, as this causes statically-generated pages to be served by the Netlify CDN
-before any middleware can be run.
+Because the middleware runs at the origin, it is run _after_ Netlify rewrites and redirects. If a static file is served
+by the Netlify CDN then the middleware is never run, as middleware only runs when a page is served by Next.js. This
+means that middleware should not be used with the `EXPERIMENTAL_MOVE_STATIC_FILES` option, as this causes
+statically-generated pages to be served by the Netlify CDN before any middleware can be run.
 
-There is currently [a bug in Next.js](https://github.com/vercel/next.js/issues/31179) that causes a proxy loop if you
-try to rewrite to a URL with a host other than localhost. If you are using a pattern like this:
+There is a bug in Next.js `<=12.0.3` that causes a proxy loop if you try to rewrite to a URL with a host other than
+localhost. This bug is fixed in version `12.0.4`. If you are using Next.js `<=12.0.3`, you can work around the bug by
+ensuring that when rewriting a request you either use a relative path, or manually set the host to `localhost` if
+returning a URL object. For example:
 
 ```typescript
 export function middleware(req: NextRequest) {
-  // Change the `nextUrl` property in some way
-  req.nextUrl = req.nextUrl.replace('something', 'somethingelse')
-  // ...then rewrite to the changed URL
-  return NextResponse.rewrite(req.nextUrl)
-}
-```
+  const rewrittenUrl = req.nextUrl
 
-...then you need to set the `nextUrl.host` to `localhost`:
+  // Change the URL in some way
+  // ...
 
-```typescript
-export function middleware(req: NextRequest) {
-  // Change the `nextUrl` property in some way
-  req.nextUrl = req.nextUrl.replace('something', 'somethingelse')
-  req.nextUrl.host = 'localhost'
+  // Before returning the URL, set the host to localhost
+  rewrittenUrl.host = 'localhost'
 
   // ...then rewrite to the changed URL
-  return NextResponse.rewrite(req.nextUrl)
+  return NextResponse.rewrite(rewrittenUrl)
 }
 ```
 
-If you have an issue with Next.js middleware on Netlify while it is beta, particularly if the issue cannot be reproduced when
-running locally, then please add a comment to [the Next plugin beta discussion](https://ntl.fyi/next-beta-feedback).
+If you have an issue with Next.js middleware on Netlify while it is beta, particularly if the issue cannot be reproduced
+when running locally, then please add a comment to
+[the Next plugin beta discussion](https://ntl.fyi/next-beta-feedback).