docs: updated webhook documentation.

docs/integrations/webhook.rst

@@ -1,22 +1,24 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.
+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}`
 ${repo_name} will be replaced with the name of repository which events is
 triggered from. Some of the variables like
 `${branch}` will result in webhook be called multiple times when multiple
 branches are pushed.
 
-Some of the variables like `${pull_request_id}` will be replaced only in
-the pull request related events.
+Starting from 4.8.0 also repository extra fields can be used. A format to use
+them is `${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
 :ref:`creating-integrations` for additional instructions.