[doc] Explain custom domain deletion requirements

src/pages/manage/reverse-proxy/custom-domains.mdx

@@ -134,11 +134,11 @@ If a custom domain returns to **Pending Verification** status - for example, aft
 
 ### Deleting a custom domain
 
-To remove a custom domain, click the delete action next to the domain in the list. Deleting a custom domain removes it from your account and makes it unavailable for any new services. Built-in domains (Free or Cluster) cannot be deleted.
+Before removing a custom domain, delete or move every service that uses the domain or any of its subdomains. Disabled services also count as dependencies. For example, a service using `app.proxy.example.com` prevents deletion of `proxy.example.com`.
 
-<Note>
-    Before deleting a custom domain, make sure no active services are using it. Services referencing a deleted domain will no longer route traffic correctly.
-</Note>
+Once no services depend on the domain, click the delete action next to it in the list. Built-in domains (Free or Cluster) cannot be deleted.
+
+If services still reference the domain, NetBird refuses deletion and the API returns HTTP `412 Precondition Failed`. The domain and its services remain unchanged, and no domain deletion activity event is recorded.
 
 ## Using domains with services