Correct reason for Turbopack incompat, remove inaccurate "good to know" (vercel#72467)

- vercel#72241 (comment)

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### What?

1. Fix the incorrect statement added in vercel#72241, as per @timneutkens'
comment here:
   - vercel#72241 (comment)
2. Remove the outdated "good to know" section stating that Turbopack
requires `mdxjs-rs` and is not compatible with `@next/mdx`

### Why?

The docs are currently incorrect

### How?

Edit [remark and rehype
plugins](https://nextjs.org/docs/app/building-your-application/configuring/mdx#remark-and-rehype-plugins)
section on docs page

---------

Co-authored-by: Tim Neutkens <[email protected]>

Author: karlhorky

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

@@ -649,7 +649,7 @@ export default withMDX(nextConfig)
 
 > **Good to know**:
 >
-> remark and rehype plugins cannot be used with [Turbopack](/docs/architecture/turbopack), due to lack of plugins support in [`mdxRs`](#using-the-rust-based-mdx-compiler-experimental)
+> remark and rehype plugins cannot be used with [Turbopack](/docs/architecture/turbopack) yet, due to [inability to pass JavaScript functions to Rust](https://github.com/vercel/next.js/issues/71819#issuecomment-2461802968)
 
 ## Remote MDX
 
@@ -786,10 +786,6 @@ module.exports = withMDX({
 })
 ```
 
-> **Good to know**:
->
-> This option is required when processing markdown and MDX while using [Turbopack](/docs/architecture/turbopack) (`next dev --turbopack`).
-
 ## Helpful Links
 
 - [MDX](https://mdxjs.com)