docs: add wait-until non streaming (#11739)

Co-authored-by: Tom Lienard <tom.lienrd@gmail.com>

errors/wait-until-non-streaming.md

@@ -0,0 +1,19 @@
+# The waitUntil API has limited support for non-streaming functions
+
+## Why This Warning Occurred
+
+The [waitUntil](https://vercel.com/docs/functions/functions-api-reference#waituntil) method allows you to enqueue asynchronous tasks to be performed during the lifecycle of the request, and these tasks can continue to resolve even after the response has been written.
+
+This is possible because `waitUntil` takes advantage of [streaming functions](https://vercel.com/docs/functions/streaming#streaming-functions).
+
+When `waitUntil` is used in a non-streaming function, the behavior is limited and the response is delayed to be written after the enqueued asynchronous tasks have been resolved.
+
+## How to Fix It
+
+To take advantage of waitUntil without compromise, all you have to do is explicitly opt-in your functions to support streaming:
+
+```js
+export const config = {
+  supportsResponseStreaming: true,
+};
+```