docs: note implications for authentication (#10713)

load functions
related to #6315

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

@@ -624,6 +624,20 @@ To summarize, a `load` function will rerun in the following situations:
 
 Note that rerunning a `load` function will update the `data` prop inside the corresponding `+layout.svelte` or `+page.svelte`; it does _not_ cause the component to be recreated. As a result, internal state is preserved. If this isn't what you want, you can reset whatever you need to reset inside an [`afterNavigate`](modules#$app-navigation-afternavigate) callback, and/or wrap your component in a [`{#key ...}`](https://svelte.dev/docs#template-syntax-key) block.
 
+## Implications for authentication
+
+A couple features of loading data have important implications for auth checks:
+- Layout `load` functions do not run on every request, such as during client side navigation between child routes. [(When do load functions rerun?)](load#rerunning-load-functions-when-do-load-functions-rerun)
+- Layout and page `load` functions run concurrently unless `await parent()` is called. If a layout `load` throws, the page `load` function runs, but the client will not receive the returned data.
+
+There are a few possible strategies to ensure an auth check occurs before protected code.
+
+To prevent data waterfalls and preserve layout `load` caches:
+- Use [hooks](hooks) to protect multiple routes before any `load` functions run
+- Use auth guards directly in `+page.server.js` `load` functions for route specific protection
+
+Putting an auth guard in `+layout.server.js` requires all child pages to call `await parent()` before protected code. Unless every child page depends on returned data from `await parent()`, the other options will be more performant.
+
 ## Further reading
 
 - [Tutorial: Loading data](https://learn.svelte.dev/tutorial/page-data)