Add info about multiple targets support for Slack integration (#37371)

* Add info about multiple targets

* Blank line before the list

---------

Co-authored-by: John Suarez <suarezjg@gmail.com>

source/_integrations/slack.markdown

@@ -106,7 +106,7 @@ sequence:
   - action: notify.YOUR_SLACK_TEAM
     data:
       message: "Fallback Text"
-      target: "#test-channel"
+      target: "#test-channel" # Single channel target
       title: "Reminder"
       data:
         blocks:
@@ -122,6 +122,38 @@ Update the blocks array with valid Slack blocks. The easiest way to create this
 
 Create a duplicate of this script to use for different messages, and different channels (the door was opened in #security, the light was left on on #lights, etc).
 
+You can also send messages to multiple targets (channels and/or users) at once:
+
+```yaml
+alias: "Notify: Multiple Targets Message"
+sequence:
+  - action: notify.YOUR_SLACK_TEAM
+    data:
+      message: "Alert: Motion detected!"
+      target: 
+        - "#security"    # Channel by name
+        - "C01234ABCD"   # Channel by ID
+        - "U5678EFGH"    # Direct message to user by ID
+      title: "Security Alert"
+      data:
+        blocks:
+          - type: section
+            text:
+              type: mrkdwn
+              text: "Motion detected in the backyard camera"
+```
+
+### Target Types
+
+The `target` field accepts either a single value or a list of values. Each target can be:
+
+- A channel name with a `#` prefix (e.g., `#general`)
+- A channel name without the `#` prefix (e.g., `general`)
+- A channel ID (e.g., `C01234ABCD`)
+- A user ID for direct messages (e.g., `U5678EFGH`)
+
+When sending files, make sure you have the proper permissions set up as described in the Setup section.
+
 ### Icons
 
 Slack uses the standard emoji sets used [here](https://slack.com/intl/en-gb/help/articles/202931348-Use-emoji-and-reactions#add-emoji-to-your-messages). Alternatively a publicly accessible URL may be used.