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>
