Port #4418 to serverless (#4445)

docs/en/serverless/alerting/create-custom-threshold-alert-rule.mdx

@@ -110,7 +110,33 @@ If the `Host A, Architecture A` group matches the rule conditions, but the `Host
 If you select one field—for example, `host.name`—and `Host A` matches the conditions but `Host B` doesn't, one alert is triggered for `Host A`.
 If both groups match the conditions, alerts are triggered for both groups.
 
-When you select **Alert me if a group stops reporting data**, the rule is triggered if a group that previously reported metrics does not report them again over the expected time period.
+## Trigger "no data" alerts (optional)
+
+Optionally configure the rule to trigger an alert when:
+
+* there is no data, or
+* a group that was previously detected stops reporting data.
+
+To do this, select **Alert me if there's no data**.
+
+The behavior of the alert depends on whether any **group alerts by** fields are specified:
+
+* **No "group alerts by" fields**: (Default) A "no data" alert is triggered if the condition fails to report data over the expected time period, or the rule fails to query ((es)). This alert means that something is wrong and there is not enough data to evaluate the related threshold.
+
+* **Has "group alerts by" fields**: If a previously detected group stops reporting data, a "no data" alert is triggered for the missing group.
+
+  For example, consider a scenario where `host.name` is the **group alerts by** field for CPU usage above 80%. The first time the rule runs, two hosts report data: `host-1` and `host-2`. The second time the rule runs, `host-1` does not report any data, so a "no data" alert is triggered for `host-1`. When the rule runs again, if `host-1` starts reporting data again, there are a couple possible scenarios:
+
+    * If `host-1` reports data for CPU usage and it is above the threshold of 80%, no new alert is triggered.
+    Instead the existing alert changes from "no data" to a triggered alert that breaches the threshold.
+    Keep in mind that no notifications are sent in this case because there is still an ongoing issue.
+    * If `host-1` reports CPU usage below the threshold of 80%, the alert status is changed to recovered.
+
+<DocCallOut title="How to untrack decommissioned hosts">
+    If a host (for example, `host-1`) is decommissioned, you probably no longer want to see "no data" alerts about it.
+    To mark an alert as untracked:
+    Go to the Alerts table, click the <DocIcon type="boxesHorizontal" size="m" title="More actions" /> icon to expand the "More actions" menu, and click *Mark as untracked*.
+</DocCallOut>
 
 ## Add actions