Adds header to make it more clear how to disable account requests (#800)

Follow up to [this
discussion](https://sourcegraph.slack.com/archives/C05EMJM2SLR/p1731436302493959)

Author: noah-berman

docs/admin/auth/builtin.mdx

@@ -57,6 +57,8 @@ New users can submit a form and site admins will see the request in the navbar o
 
 ![List of pending access requests for admin to approve](https://storage.googleapis.com/sourcegraph-assets/docs/images/admin/auth/pending-access-requests.png)
 
+## Disable account requests from users
+
 The account request feature can be disabled by setting `"auth.accessRequest": { "enabled": false }`. When disabled, new user accounts can only be created by the site admin manually.
 
 ## Creating builtin authentication users