docs(mdx): add clarification for mdxRs (#67237)

## Why?

Add clarification that you need the experimental config:

```
module.exports = withMDX({
  experimental: {
    mdxRs: true,
  },
})
```

when using [Turbopack](https://nextjs.org/docs/architecture/turbopack).

---------

Co-authored-by: Delba de Oliveira <32464864+delbaoliveira@users.noreply.github.com>

docs/02-app/01-building-your-application/07-configuring/05-mdx.mdx

@@ -782,6 +782,10 @@ module.exports = withMDX({
 })
 ```
 
+> **Good to know**:
+>
+> This option is required when processing markdown and MDX while using [Turbopack](/docs/architecture/turbopack) (`next dev --turbo`).
+
 ## Helpful Links
 
 - [MDX](https://mdxjs.com)