Adjust Send on all alerts to default label (#16554)

Alerting: Adjust label for send on all alerts to default 

docs/sources/alerting/notifications.md

@@ -27,7 +27,7 @@ can configure and setup a new Notification Channel.
 You specify a name and a type, and type specific options. You can also test the notification to make
 sure it's setup correctly.
 
-### Send on all alerts
+### Default (send on all alerts)
 
 When checked, this option will notify for all alert rules - existing and new.
 

public/app/features/alerting/partials/notification_edit.html

@@ -20,7 +20,7 @@
       </div>
       <gf-form-switch
           class="gf-form"
-          label="Send on all alerts"
+          label="Default (send on all alerts)"
           label-class="width-14"
           checked="ctrl.model.isDefault"
           tooltip="Use this notification for all alerts">