docs(use-hydration): document fallback hydration behavior when plugin is not installed

Author: johnleider

apps/docs/src/pages/composables/plugins/use-hydration.md

@@ -89,6 +89,19 @@ All properties are `Readonly<ShallowRef>` and update when the root component mou
 | `isHydrated` | `ShallowRef<boolean>` | True after root component mounts |
 | `isSettled` | `ShallowRef<boolean>` | True after next tick post-hydration |
 
+## Fallback Hydration
+
+When `useHydration()` is called without the plugin installed (no `createHydrationPlugin` in your app), it returns a fallback context where `isHydrated` and `isSettled` are **immediately `true`**. This means components that conditionally render based on `isHydrated` work correctly in client-only apps without any plugin setup.
+
+```ts no-filename
+// Without plugin: isHydrated.value === true immediately
+// With plugin:    isHydrated.value starts false, becomes true after mount
+const { isHydrated } = useHydration()
+```
+
+> [!TIP]
+> The fallback is also what SSR-aware composables like `useResizeObserver` use internally — they call `useHydration()` to defer observation until after hydration, but work correctly even in environments where the plugin isn't installed.
+
 ## Examples
 
 ::: example