docs: more detailed error message when failing to parse a middleware …

…matcher (vercel#41390)

## 📖 What's in there?

This is a follow-up for vercel#40180
Someone made the good point that users may get directed to this error page while setting up middleware matchers. And that page has no information (yet) about it.

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [x] 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: Balázs Orbán <18369201+balazsorban44@users.noreply.github.com>

errors/invalid-route-source.md

@@ -2,12 +2,18 @@
 
 #### Why This Error Occurred
 
-When defining custom routes a route was added that causes an error during parsing. This can be due to trying to use normal `RegExp` syntax like negative lookaheads (`?!exclude`) without following `path-to-regexp`'s syntax for it.
+When defining custom routes, or a middleware `matcher`, a pattern could not be parsed.
+
+This could have been due to trying to use normal `RegExp` syntax like negative lookaheads (`?!exclude`) without following [`path-to-regexp`](https://github.com/pillarjs/path-to-regexp)'s syntax for it.
 
 #### Possible Ways to Fix It
 
 Wrap the `RegExp` part of your `source` as an un-named parameter.
 
+---
+
+Custom routes:
+
 **Before**
 
 ```js
@@ -26,6 +32,26 @@ Wrap the `RegExp` part of your `source` as an un-named parameter.
 }
 ```
 
+---
+
+Middleware:
+
+**Before**
+
+```js
+const config = {
+  matcher: '/feedback/(?!general)',
+}
+```
+
+**After**
+
+```js
+const config = {
+  matcher: '/feedback/((?!general).*)',
+}
+```
+
 ### Useful Links
 
 - [path-to-regexp](https://github.com/pillarjs/path-to-regexp)