Update configure-custom-domain.md

Managed cert is restricted to only public domain names

Author: dlepow

articles/api-management/configure-custom-domain.md

@@ -105,6 +105,7 @@ API Management offers a free, managed TLS certificate for your domain, if you do
 * Not supported in the following Azure regions: France South and South Africa West
 * Currently available only in the Azure cloud
 * Does not support root domain names (for example, `contoso.com`). Requires a fully qualified name such as `api.contoso.com`.
+* Supports only public domain names
 * Can only be configured when updating an existing API Management instance, not when creating an instance