docs: Middleware explicit runtime differences (#83348)

In previous docs versions, while not explicitly stated, it was rather
clearer that middleware has its own runtime.

Also, people who have used bundle-analyzer would for example be familiar
with the 3 bundles, or those who have worked with
`next.config.js`/instrumentation would understand that there's some
different runtimes at play going on there, but those who exclusively
work with middleware might still miss on this detail.

---------

Co-authored-by: JJ Kasper <[email protected]>

Author: icyJoseph

docs/01-app/03-api-reference/03-file-conventions/middleware.mdx

@@ -12,6 +12,12 @@ The `middleware.js|ts` file is used to write [Middleware](/docs/app/api-referenc
 
 Middleware executes before routes are rendered. It's particularly useful for implementing custom server-side logic like authentication, logging, or handling redirects.
 
+> **Good to know**:
+>
+> Middleware is meant to be invoked separately of your render code and in optimized cases deployed to your CDN for fast redirect/rewrite handling, you should not attempt relying on shared modules or globals.
+>
+> To pass information from Middleware to your application, use [headers](#setting-headers), [cookies](#using-cookies), [rewrites](/docs/app/api-reference/functions/next-response#rewrite), [redirects](/docs/app/api-reference/functions/next-response#redirect), or the URL.
+
 Create a `middleware.ts` (or `.js`) file in the project root, or inside `src` if applicable, so that it is located at the same level as `pages` or `app`.
 
 ```tsx filename="middleware.ts" switcher
@@ -281,7 +287,7 @@ export function middleware(request) {
 }
 ```
 
-## Using Cookies
+### Using Cookies
 
 Cookies are regular headers. On a `Request`, they are stored in the `Cookie` header. On a `Response` they are in the `Set-Cookie` header. Next.js provides a convenient way to access and manipulate these cookies through the `cookies` extension on `NextRequest` and `NextResponse`.