docs(useWebNotification): add permission tip and update example (vueuse#4660)

Co-authored-by: Robin <robin.kehl@singular-it.de>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: 3 people

packages/core/useWebNotification/index.md

@@ -10,10 +10,15 @@ The Web Notification interface of the Notifications API is used to configure and
 
 ## Usage
 
+::: tip
+Before an app can send a notification, the user must grant the application the right to do so. The user's OS settings may also prevent expected notification behaviour.
+:::
+
 ```ts
 const {
   isSupported,
   notification,
+  permissionGranted,
   show,
   close,
   onClick,
@@ -28,7 +33,7 @@ const {
   tag: 'test',
 })
 
-if (isSupported.value)
+if (isSupported.value && permissionGranted.value)
   show()
 ```