docs: clarify what handleFetch affects (#13788)

* clarify what handleFetch affects

* edit function message too

* changeset

* tweak wording

* reword to avoid the subhed

---------

Co-authored-by: Rich Harris <[email protected]>

Author: teemingc

.changeset/old-kangaroos-jog.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+chore: clarify which functions `handleFetch` affects

documentation/docs/30-advanced/20-hooks.md

@@ -106,7 +106,7 @@ Note that `resolve(...)` will never throw an error, it will always return a `Pro
 
 ### handleFetch
 
-This function allows you to modify (or replace) a `fetch` request that happens inside a `load`, `action` or `handle` function that runs on the server (or during prerendering).
+This function allows you to modify (or replace) the result of an [`event.fetch`](load#Making-fetch-requests) call that runs on the server (or during prerendering) inside an endpoint, `load`, `action`, `handle`, `handleError` or `reroute`.
 
 For example, your `load` function might make a request to a public URL like `https://api.yourapp.com` when the user performs a client-side navigation to the respective page, but during SSR it might make sense to hit the API directly (bypassing whatever proxies and load balancers sit between it and the public internet).
 
@@ -126,13 +126,9 @@ export async function handleFetch({ request, fetch }) {
 }
 ```
 
-**Credentials**
+Requests made with `event.fetch` follow the browser's credentials model — for same-origin requests, `cookie` and `authorization` headers are forwarded unless the `credentials` option is set to `"omit"`. For cross-origin requests, `cookie` will be included if the request URL belongs to a subdomain of the app — for example if your app is on `my-domain.com`, and your API is on `api.my-domain.com`, cookies will be included in the request.
 
-For same-origin requests, SvelteKit's `fetch` implementation will forward `cookie` and `authorization` headers unless the `credentials` option is set to `"omit"`.
-
-For cross-origin requests, `cookie` will be included if the request URL belongs to a subdomain of the app — for example if your app is on `my-domain.com`, and your API is on `api.my-domain.com`, cookies will be included in the request.
-
-If your app and your API are on sibling subdomains — `www.my-domain.com` and `api.my-domain.com` for example — then a cookie belonging to a common parent domain like `my-domain.com` will _not_ be included, because SvelteKit has no way to know which domain the cookie belongs to. In these cases you will need to manually include the cookie using `handleFetch`:
+There is one caveat: if your app and your API are on sibling subdomains — `www.my-domain.com` and `api.my-domain.com` for example — then a cookie belonging to a common parent domain like `my-domain.com` will _not_ be included, because SvelteKit has no way to know which domain the cookie belongs to. In these cases you will need to manually include the cookie using `handleFetch`:
 
 ```js
 /// file: src/hooks.server.js

packages/kit/src/exports/public.d.ts

@@ -790,7 +790,7 @@ export type HandleClientError = (input: {
 }) => MaybePromise<void | App.Error>;
 
 /**
- * The [`handleFetch`](https://svelte.dev/docs/kit/hooks#Server-hooks-handleFetch) hook allows you to modify (or replace) a `fetch` request that happens inside a `load` function that runs on the server (or during prerendering).
+ * The [`handleFetch`](https://svelte.dev/docs/kit/hooks#Server-hooks-handleFetch) hook allows you to modify (or replace) the result of an [`event.fetch`](https://svelte.dev/docs/kit/load#Making-fetch-requests) call that runs on the server (or during prerendering) inside an endpoint, `load`, `action`, `handle`, `handleError` or `reroute`.
  */
 export type HandleFetch = (input: {
 	event: RequestEvent;

packages/kit/types/index.d.ts

@@ -772,7 +772,7 @@ declare module '@sveltejs/kit' {
 	}) => MaybePromise<void | App.Error>;
 
 	/**
-	 * The [`handleFetch`](https://svelte.dev/docs/kit/hooks#Server-hooks-handleFetch) hook allows you to modify (or replace) a `fetch` request that happens inside a `load` function that runs on the server (or during prerendering).
+	 * The [`handleFetch`](https://svelte.dev/docs/kit/hooks#Server-hooks-handleFetch) hook allows you to modify (or replace) the result of an [`event.fetch`](https://svelte.dev/docs/kit/load#Making-fetch-requests) call that runs on the server (or during prerendering) inside an endpoint, `load`, `action`, `handle`, `handleError` or `reroute`.
 	 */
 	export type HandleFetch = (input: {
 		event: RequestEvent;