docs: clarify some +error.svelte behavior (#8485)

* docs: clarify some +error.svelte behavior

closes #7875
closes #8349

* Apply suggestions from code review

Co-authored-by: Rich Harris <richard.a.harris@gmail.com>

documentation/docs/20-core-concepts/10-routing.md

@@ -129,6 +129,10 @@ If an error occurs during `load`, SvelteKit will render a default error page. Yo
 
 SvelteKit will 'walk up the tree' looking for the closest error boundary — if the file above didn't exist it would try `src/routes/blog/+error.svelte` and then `src/routes/+error.svelte` before rendering the default error page. If _that_ fails (or if the error was thrown from the `load` function of the root `+layout`, which sits 'above' the root `+error`), SvelteKit will bail out and render a static fallback error page, which you can customise by creating a `src/error.html` file.
 
+If the error occurs inside a `load` function in `+layout(.server).js`, the closest error boundary in the tree is an `+error.svelte` file _above_ that layout (not next to it).
+
+If no route can be found (404), `src/routes/+error.svelte` (or the default error page, if that file does not exist) will be used.
+
 > `+error.svelte` is _not_ used when an error occurs inside [`handle`](/docs/hooks#server-hooks-handle) or a [+server.js](#server) request handler.
 
 You can read more about error handling [here](/docs/errors).

documentation/docs/30-advanced/25-errors.md

@@ -131,7 +131,7 @@ You can customise the fallback error page by adding a `src/error.html` file:
 
 SvelteKit will replace `%sveltekit.status%` and `%sveltekit.error.message%` with their corresponding values.
 
-If the error instead occurs inside a `load` function while rendering a page, SvelteKit will render the [`+error.svelte`](/docs/routing#error) component nearest to where the error occurred.
+If the error instead occurs inside a `load` function while rendering a page, SvelteKit will render the [`+error.svelte`](/docs/routing#error) component nearest to where the error occurred. If the error occurs inside a `load` function in `+layout(.server).js`, the closest error boundary in the tree is an `+error.svelte` file _above_ that layout (not next to it).
 
 The exception is when the error occurs inside the root `+layout.js` or `+layout.server.js`, since the root layout would ordinarily _contain_ the `+error.svelte` component. In this case, SvelteKit uses the fallback error page.