docs: Improve transport hook docs (#13996)

Author: hamboomger

documentation/docs/20-core-concepts/20-load.md

@@ -206,7 +206,7 @@ Universal `load` functions are called with a `LoadEvent`, which has a `data` pro
 
 A universal `load` function can return an object containing any values, including things like custom classes and component constructors.
 
-A server `load` function must return data that can be serialized with [devalue](https://github.com/rich-harris/devalue) — anything that can be represented as JSON plus things like `BigInt`, `Date`, `Map`, `Set` and `RegExp`, or repeated/cyclical references — so that it can be transported over the network. Your data can include [promises](#Streaming-with-promises), in which case it will be streamed to browsers.
+A server `load` function must return data that can be serialized with [devalue](https://github.com/rich-harris/devalue) — anything that can be represented as JSON plus things like `BigInt`, `Date`, `Map`, `Set` and `RegExp`, or repeated/cyclical references — so that it can be transported over the network. Your data can include [promises](#Streaming-with-promises), in which case it will be streamed to browsers. If you need to serialize/deserialize custom types, use [transport hooks](https://svelte.dev/docs/kit/hooks#Universal-hooks-transport).
 
 ### When to use which
 

packages/kit/src/runtime/server/utils.js

@@ -133,7 +133,10 @@ export function redirect_response(status, location) {
  */
 export function clarify_devalue_error(event, error) {
 	if (error.path) {
-		return `Data returned from \`load\` while rendering ${event.route.id} is not serializable: ${error.message} (${error.path})`;
+		return (
+			`Data returned from \`load\` while rendering ${event.route.id} is not serializable: ${error.message} (${error.path}). ` +
+			`If you need to serialize/deserialize custom types, use transport hooks: https://svelte.dev/docs/kit/hooks#Universal-hooks-transport.`
+		);
 	}
 
 	if (error.path === '') {

packages/kit/test/apps/basics/test/cross-platform/test.js

@@ -220,7 +220,8 @@ test.describe('Shadowed pages', () => {
 
 			expect(await page.textContent('h1')).toBe('500');
 			expect(await page.textContent('#message')).toBe(
-				'This is your custom error page saying: "Data returned from `load` while rendering /shadowed/serialization is not serializable: Cannot stringify arbitrary non-POJOs (data.nope) (500 Internal Error)"'
+				'This is your custom error page saying: "Data returned from `load` while rendering /shadowed/serialization is not serializable: Cannot stringify arbitrary non-POJOs (data.nope).' +
+					' If you need to serialize/deserialize custom types, use transport hooks: https://svelte.dev/docs/kit/hooks#Universal-hooks-transport. (500 Internal Error)"'
 			);
 		});
 	}