docs: sync slack integration docs

docs/services/slack.md

@@ -39,7 +39,7 @@ The Slack notification service configuration includes following settings:
     ```
 
 1. Define service type slack in data section of `argocd-notifications-cm` configmap:
-service
+
     ```yaml
     apiVersion: v1
     kind: ConfigMap
@@ -60,6 +60,26 @@ service
         notifications.argoproj.io/subscribe.on-sync-succeeded.slack: my_channel
     ```
 
+1. Annotation with more than one trigger multiple of destinations and recipients
+
+    ```yaml
+    apiVersion: argoproj.io/v1alpha1
+    kind: Application
+    metadata:
+    annotations:
+      notifications.argoproj.io/subscriptions: |
+        - trigger: [on-scaling-replica-set, on-rollout-updated, on-rollout-step-completed]
+          destinations:
+            - service: slack
+              recipients: [my-channel-1, my-channel-2]
+            - service: email
+              recipients: [recipient-1, recipient-2, recipient-3 ]
+        - trigger: [on-rollout-aborted, on-analysis-run-failed, on-analysis-run-error]
+          destinations:
+            - service: slack
+              recipients: [my-channel-21, my-channel-22]
+    ```
+
 ## Templates
 
 Notification templates can be customized to leverage slack message blocks and attachments
@@ -145,3 +165,5 @@ template.app-sync-failed: |
     groupingKey: "{{.app.status.sync.revision}}"
     notifyBroadcast: true
 ```
+
+The message is sent according to the `deliveryPolicy` string field under the `slack` field. The available modes are `Post` (default), `PostAndUpdate`, and `Update`. The `PostAndUpdate` and `Update` settings require `groupingKey` to be set.