docs: updated webhook documentation.

docs/integrations/webhook.rst

@@ -3,9 +3,9 b''
 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.
+The :ref:`creating-integrations` 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/${repo_name}`
@@ -14,8 +14,10 b' triggered from. Some of the variables li'
 `${branch}` will result in webhook be called multiple times when multiple
 branches are pushed.
 
-Some of the variables like `${pull_request_id}` will be replaced only in
-the pull request related events.
+Starting from 4.8.0 also repository extra fields can be used. A format to use
+them is `${extra:field_key}`. It's usefull to use them to specify custom
+repo only parameters. 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 any previous custom webhook created. See