Update docs for the slack delivery method (#495)

Author: TheZero0-ctrl

docs/delivery_methods/slack.md

@@ -1,19 +1,18 @@
-### Slack Delivery Method
-
-Sends a Slack notification via webhook.
-
-`deliver_by :slack`
-
-##### Options
-
-* `format: :format_for_slack` - *Optional*
-
-  Use a custom method to define the payload sent to Slack. Method should return a Hash.
-
-* `url: :url_for_slack` - *Optional*
-
-  Use a custom method to retrieve the Slack Webhook URL. Method should return a String.
-
-  Defaults to `Rails.application.credentials.slack[:notification_url]`
-
-
+# Slack Delivery Method
+
+Send a Slack message to notify users in a channel.
+
+## Usage
+
+```ruby
+class CommentNotification
+  deliver_by :slack do |config|
+    config.url = "https://slack.com..."
+    config.json = -> {
+      {
+        # ...
+      }
+    }
+  end
+end
+```