Clarify a custom server doesn't mean you can't use an existing backend. (#23200)

I've been this confusion quite a few times.

> So with my requirements, how big of a sin is it to use Next only for the frontend and get all its data, JWT tokens, etc. from my NodeJS backend by calling its respective APIs to said backend?

> Even the docs mention that running a custom server is not a great idea... 

docs/advanced-features/custom-server.md

@@ -15,7 +15,7 @@ description: Start a Next.js app programmatically using a custom server.
   </ul>
 </details>
 
-Typically you start your next server with `next start`. It's possible, however, to start a server 100% programmatically in order to use custom route patterns.
+By default, Next.js includes its own server with `next start`. If you have an existing backend, you can still use it with Next.js (this is not a custom server). A custom Next.js server allows you to start a server 100% programmatically in order to use custom server patterns. Most of the time, you will not need this – but it's available for complete customization.
 
 > A custom server **can not** be deployed on [Vercel](https://vercel.com/solutions/nextjs), the platform Next.js was made for.