Document device sleep and persistence behaviour of alarms API

site/en/docs/extensions/reference/alarms/index.md

@@ -17,6 +17,20 @@ To use the `chrome.alarms` API, declare the `"alarms"` permission in the [manife
 }
 ```
 
+## Concepts and usage
+
+### Device sleep
+
+The behavior of alarms when a device goes to sleep is currently undefined. An alarm will never fire
+early but may fire significantly later than expected if a device went to sleep after an alarm was
+scheduled.
+
+### Persistence
+
+Alarms generally persist until an extension is updated. However, this is not guarenteed and alarms
+may be cleared when the browser is restarted. Consequently, consider storing any important alarms in
+storage and making sure they exist each time your service worker starts up.
+
 ## Examples
 
 The following examples show how to use and respond to an alarm. To try this API,