feat: document locals in adapter API

src/content/docs/en/reference/adapter-reference.mdx

@@ -136,14 +136,30 @@ export function start(manifest) {
 
 The following methods are provided:
 
-##### `app.render(request)`
+##### `app.render(request, routeData, locals)`
 
-This method calls the Astro page that matches the request, renders it, and returns a Promise to a [Response](https://developer.mozilla.org/en-US/docs/Web/API/Response) object. This also works for API routes, that do not render pages.
+This method calls the Astro page that matches the request, renders it, and returns a Promise to a [Response](https://developer.mozilla.org/en-US/docs/Web/API/Response) object. This also works for API routes that do not render pages.
 
 ```js
 const response = await app.render(request);
 ```
 
+The method accepts a mandatory `request` argument, and two other optionals arguments: [`routeData`](en/reference/integrations-reference/#routedata-type-reference) and [`locals`](https://docs.astro.build/en/guides/middleware/#locals).
+
+The example below reads a header named `x-private-header`, which attempts to parse it as an object and pass it to `locals`, which can then be passed to any [middleware function](/en/guides/middleware/).
+
+```js
+const privateHeader = request.headers.get("x-private-header");
+let locals = {};
+try {
+    if (privateHeader) {
+        locals = JSON.parse(privateHeader);
+    }
+} finally {
+    const response = await app.render(request, undefined, locals);
+}
+```
+
 ##### `app.match(request)`
 
 This method is used to determine if a request is matched by the Astro app's routing rules.