Update documentation with multiple channels

app/models/project_services/slack_service.rb

@@ -20,7 +20,8 @@ class SlackService < ChatNotificationService
       <li><a href="https://slack.com/apps/A0F7XDUAZ-incoming-webhooks">Add an incoming webhook</a> in your Slack team. The default channel can be overridden for each event.</li>
       <li>Paste the <strong>Webhook URL</strong> into the field below.</li>
       <li>Select events below to enable notifications. The <strong>Channel names</strong> and <strong>Username</strong> fields are optional.</li>
-    </ol>'
+    </ol>
+    <strong>Note:</strong> Usernames and private channels are not supported.'
   end
 
   def default_channel_placeholder

doc/user/project/integrations/slack.md

@@ -16,7 +16,7 @@ The Slack Notifications Service allows your GitLab project to send events (e.g.
 1. Select the **Slack notifications** project service to configure it.
 1. Check the **Active** checkbox to turn on the service.
 1. Check the checkboxes corresponding to the GitLab events you want to send to Slack as a notification.
-1. For each event, optionally enter the Slack channel where you want to send the event. (Do _not_ include the `#` symbol.) If left empty, the event will be sent to the default channel that you configured in the Slack Configuration step.
+1. For each event, optionally enter the Slack channels where you want to send the event, separated by a comma. If left empty, the event will be sent to the default channel that you configured in the Slack Configuration step. **Note:** Usernames and private channels are not supported. To send direct messages, use the Member ID found under user's Slack profile.
 1. Paste the **Webhook URL** that you copied from the Slack Configuration step.
 1. Optionally customize the Slack bot username that will be sending the notifications.
 1. Configure the remaining options and click `Save changes`.