Update the serverFunctions.onError reference

Author: adipascu

src/routes/solid-start/v2/reference/config/solid-start.mdx

@@ -139,7 +139,17 @@ solidStart({
 
 Path to a module whose default export handles values thrown by server functions before SolidStart serializes them into a response.
 
-The handler can report the original value and return a safer replacement for the client. Return `undefined` to preserve the original value. The module is bundled only into the server, so it can import server-only code.
+The export is called synchronously with the thrown value, and its return value is not awaited, so it cannot be an `async` function.
+
+- Return `undefined` or `null` to preserve the original value.
+- Return a `Response` to have it handled as if the server function had thrown it.
+- Return any other value to send it in place of the original. It reaches the client serialized along with its own properties, so a replacement meant to be safe must not carry internal detail.
+
+Control flow arrives the same way errors do: a [thrown `redirect`](/solid-start/v2/advanced/return-responses) reaches the export as a `Response`. Return those unchanged, or a handler that replaces everything it sees turns redirects into errors.
+
+Only server function calls made over the network reach the export. A server function called during rendering runs in process and throws straight to its caller, and errors from API routes never reach it either.
+
+The module is bundled only into the server, so it can import server-only code.
 
 ```tsx title="vite.config.ts"
 solidStart({
@@ -153,13 +163,14 @@ solidStart({
 import type { ServerFunctionErrorHandler } from "@solidjs/start/server";
 
 const onServerFunctionError: ServerFunctionErrorHandler = (thrown) => {
-	console.error(thrown);
-
-	if (thrown instanceof Error) {
-		return new Error("The server function failed.");
+	// `redirect` and friends throw a Response. Leave that control flow alone.
+	if (thrown instanceof Response) {
+		return thrown;
 	}
 
-	return undefined;
+	console.error(thrown);
+
+	return new Error("The server function failed.");
 };
 
 export default onServerFunctionError;