update docs

Author: Rich-Harris

documentation/docs/06-hooks.md

@@ -67,7 +67,7 @@ You can add call multiple `handle` functions with [the `sequence` helper functio
 `resolve` also supports a second, optional parameter that gives you more control over how the response will be rendered. That parameter is an object that can have the following fields:
 
 - `ssr: boolean` (default `true`) — if `false`, renders an empty 'shell' page instead of server-side rendering
-- `transformPageChunk(opts: { html: string, done: boolean }): string` — applies custom transforms to HTML. If `done` is true, it's the final chunk
+- `transformPageChunk(opts: { html: string, done: boolean }): MaybePromise<string | undefined>` — applies custom transforms to HTML. If `done` is true, it's the final chunk. Chunks are not guaranteed to be well-formed HTML (they could include an element's opening tag but not its closing tag, for example) but they will always be split at sensible boundaries such as `%sveltekit.head%` or layout/page components.
 
 ```js
 /// file: src/hooks.js