Update onboard-apex-domain.md

Explain that apex domains do not support CDN-managed certificates

Author: taraldr

articles/cdn/onboard-apex-domain.md

@@ -24,6 +24,10 @@ Mapping your apex or root domain to your CDN profile requires CNAME flattening o
 
 You can use the Azure portal to onboard an apex domain on your CDN and enable HTTPS on it by associating it with a certificate for TLS termination. Apex domains are also referred as root or naked domains.
 
+> [!IMPORTANT]
+> CDN-managed certificates are not available for root or apex domains. If your Azure CDN custom domain is a root or apex domain, you must use the Bring your own certificate feature.
+>
+
 ## Create an alias record for zone apex
 
 1. Open **Azure DNS** configuration for the domain to be onboarded.