docs: add note about variables in webhooks.

docs/integrations/webhook.rst

@@ -1,12 +1,22 b''
 .. _integrations-webhook:
 
 Webhook integration
 ===================
 
 The Webhook integration allows you to POST events such as repository pushes
 or pull requests to a custom http endpoint as a json dict with details of the
 event.
 
+Starting from 4.5.0 release, webhook integration allows to use variables
+inside the URL. For example in URL `https://server-example.com/${repo_name}`
+${repo_name} will be replaced with the name of repository which events is
+triggered from. Some of the variables like
+`${branch}` will result in webhook be called multiple times when multiple
+branches are pushed.
+
+Some of the variables like `${pull_request_id}` will be replaced only in
+the pull request related events.
+
 To create a webhook integration, select "webhook" in the integration settings
-and use the url and key from your custom webhook. See
-:ref:`creating-integrations` for additional instructions.
  No newline at end of file
+and use the URL and key from your any previous custom webhook created. See
+:ref:`creating-integrations` for additional instructions.