Sync kit docs (#818)

sync kit docs

Co-authored-by: Rich-Harris <1162160+Rich-Harris@users.noreply.github.com>

Author: github-actions[bot]

apps/svelte.dev/content/docs/kit/98-reference/10-@sveltejs-kit.md

@@ -10,6 +10,7 @@ import {
 	VERSION,
 	error,
 	fail,
+	isActionFailure,
 	isHttpError,
 	isRedirect,
 	json,
@@ -86,6 +87,22 @@ function fail<
 
 
 
+## isActionFailure
+
+Checks whether this is an action failure thrown by `fail`.
+
+<div class="ts-block">
+
+```dts
+function isActionFailure(
+	e: unknown
+): e is ActionFailure<undefined>;
+```
+
+</div>
+
+
+
 ## isHttpError
 
 Checks whether this is an error thrown by `error`.