docs: document RenderOptions.waitUntil

[gllmt]

Description (required)

Documents the new RenderOptions.waitUntil() option added by withastro/astro#16187.

This explains how adapter authors can forward a runtime waitUntil() hook to app.render() so runtime cache providers can schedule background work without blocking the response.

Related issues & labels (optional)

• Related: fix(cloudflare): pass waitUntil to cache providers astro#16187
• Suggested label: merge-on-release

For Astro version: 6.2.0. See astro PR withastro/astro#16187.

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[gllmt]

Companion docs PR for withastro/astro#16187. Since this documents a new public option that is not released yet, this should likely follow the usual merge-on-release docs flow.

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[ArmandPhilippot]

Thanks for the fix and the docs! I left a few suggestions, especially around the code snippet, as I'm not sure if it's currently useful.

And, please use our PR template the next time.

[ArmandPhilippot]

We use parentheses for functions:

Suggested change

	#### `RenderOptions.waitUntil`
	#### `RenderOptions.waitUntil()`

[ArmandPhilippot]

I'm not sure the current example is really helpful because this only lists what is available in RenderOptions. Would something like the following would be correct as minimal example?

Suggested change

	```js "waitUntil"
	return app.render(request, {
	routeData,
	locals,
	waitUntil,
	});
	```
	```js {8}
	import { createApp } from 'astro/app/entrypoint';
	const app = createApp();
	export async function handler(event, context) {
	// ...
	return app.render(event.request, {
	waitUntil: context.waitUntil.bind(context),
	});
	}
	```

[ArmandPhilippot]

See my suggestion for the code snippet:

Suggested change

	The following example forwards a runtime's `waitUntil` implementation to [`app.render()`](#apprender):
	The following example forwards a runtime's `waitUntil()` implementation to [`app.render()`](#apprender) in an [adapter server entrypoint](/en/reference/adapter-reference/#building-a-server-entrypoint):

[gllmt]

Thanks for the suggestions, that makes sense.

I’ve applied the updates:

• aligned the docs style for RenderOptions.waitUntil()
• replaced the snippet with a concrete adapter server entrypoint example
• updated the intro to reference both waitUntil() and the adapter entrypoint docs
• updated the PR body to follow the docs template

pnpm check passes.