Add sentence explaining what happens when the `for` clause is omitted (#12457)

Just adding a statement here explaining that the default is an
immediate move to "active" without a pending state.

Signed-off-by: Tim Martin <tim@timmartin.me>

docs/configuration/alerting_rules.md

@@ -32,7 +32,11 @@ groups:
 ```
 
 The optional `for` clause causes Prometheus to wait for a certain duration
-between first encountering a new expression output vector element and counting an alert as firing for this element. In this case, Prometheus will check that the alert continues to be active during each evaluation for 10 minutes before firing the alert. Elements that are active, but not firing yet, are in the pending state.
+between first encountering a new expression output vector element and counting
+an alert as firing for this element. In this case, Prometheus will check that
+the alert continues to be active during each evaluation for 10 minutes before
+firing the alert. Elements that are active, but not firing yet, are in the pending state.
+Alerting rules without the `for` clause will become active on the first evaluation.
 
 The `labels` clause allows specifying a set of additional labels to be attached
 to the alert. Any existing conflicting labels will be overwritten. The label