document where to find the secret key to make the kgb web hooks work

See team#41413

howto/gitlab.md

@@ -330,6 +330,11 @@ And, finally, list all hooks with:
 
     sudo gitlab-rake gitlab:web_hook:list
 
+The hook needs a secret token to be operational. This secret is stored
+in Puppet's Trocla database as `profile::kgb_bot::gitlab_token`:
+
+    trocla get profile::kgb_bot::gitlab_token plain
+
 ## Setting up two-factor authentication (2FA)
 
 We strongly recommend you enable two-factor authentication on