fix: Update documentation for Automatic deploy subdomains

https://answers.netlify.com/t/new-automatic-deploy-subdomains/87711/6

Author: Erik Westrup

go/models/site.go

@@ -2531,6 +2531,10 @@ definitions:
         type: array
         items:
           type: string
+      branch_deploy_custom_domain:
+        type: string
+      deploy_preview_custom_domain:
+        type: string
       password:
         type: string
       notification_email: