Add ttl feature description to Pushover documentation (#39050)

Co-authored-by: c0ffeeca7 <38767475+c0ffeeca7@users.noreply.github.com>

source/_integrations/pushover.markdown

@@ -54,6 +54,19 @@ To use a specific Pushover device, set it using `target`. If one of the entered
       sound: pianobar
       priority: 0
 ```
+Using the `ttl` parameter, messages may be set to delete automatically after a certain period of time. This is useful for messages that, at some point, outlive their usefulness. The `ttl` parameter specifies a time-to-live in seconds. In the following example, the message will self-delete from the targeted device(s) after 6 hours.
+
+```yaml
+  - service: notify.pushover
+    data:
+      message: "This is the message"
+      title: "Title of message"
+      target:
+        - pixel9
+        - johnsmith
+      data:
+        ttl: 21600 
+```
 
 To use the highest priority, which repeats the notification every x seconds (`retry`) for the duration of y seconds (`expire`), you MUST specify these parameters. The minimal time for the `retry` parameter is 30 seconds. The `expire` parameter has a maximum of 10800 seconds (3 hours). If you target more than one device, make sure to enable the advanced option "Notification dismissal sync" in the app to be able to dismiss the alert on all devices simultaneously.