[release-12.0.2] docs(alerting): document `Keep firing for` in Alert rule evaluation intro (#106213)

docs(alerting): document `Keep firing for` in Alert rule evaluation intro (#106139)

(cherry picked from commit e1400e67d3)

Co-authored-by: Pepe Cano <825430+ppcano@users.noreply.github.com>

docs/sources/alerting/fundamentals/alert-rule-evaluation/_index.md

@@ -34,10 +34,11 @@ refs:
 
 # Alert rule evaluation
 
-The criteria determining when an alert rule fires are based on two settings:
+The criteria determining when an alert rule fires are based on three settings:
 
 - [Evaluation group](#evaluation-group): how frequently the alert rule is evaluated.
 - [Pending period](#pending-period): how long the condition must be met to start firing.
+- [Keep firing for](#pending-period): how long the alert continues to fire after the condition is no longer met.
 
 {{< figure src="/media/docs/alerting/alert-rule-evaluation-2.png" max-width="750px" alt="Set the evaluation behavior of the alert rule in Grafana." caption="Set alert rule evaluation" >}}