Docs: Note when using For and No Data in alert rule (#19185)

Add a note to documentation regarding using For and No Data.

Co-Authored-By: Kyle Brandt <kyle@grafana.com>

docs/sources/alerting/rules.md

@@ -51,7 +51,9 @@ Here you can specify the name of the alert rule and how often the scheduler shou
 
 ### For
 
-> This setting is available in Grafana 5.4 and above.
+> **Important note regarding No Data:**
+>
+> Do not use `For` with the `If no data or all values are null` setting set to `No Data`. The triggering of `No Data` will trigger instantly and not take `For` into consideration. This may also result in that an OK notification not being sent if alert transitions from `No Data -> Pending -> OK`.
 
 If an alert rule has a configured `For` and the query violates the configured threshold it will first go from `OK` to `Pending`. Going from `OK` to `Pending` Grafana will not send any notifications. Once the alert rule has been firing for more than `For` duration, it will change to `Alerting` and send alert notifications.