docs: more detailed error message when failing to parse a middleware matcher (#41390)
docs: more detailed error message when failing to parse a middleware matcher (#41390)

## 📖 What's in there?

This is a follow-up for https://github.com/vercel/next.js/pull/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 <d46d8284d075dcda7077b9fe124033a6869f575e@users.noreply.github.com>
JeromeFitz/next.js,JeromeFitz/next.js,JeromeFitz/next.js,JeromeFitz/next.js