[SSL] Update troubleshooting.mdx

CUSTESC-46925

Author: ngayerie

src/content/docs/ssl/edge-certificates/changing-dcv-method/troubleshooting.mdx

@@ -53,6 +53,7 @@ Consider the following when troubleshooting:
 - [DNSSEC](https://www.cloudflare.com/learning/dns/dns-security/) must be configured correctly. You can use [DNSViz](https://dnsviz.net/) to understand and troubleshoot the deployment of DNSSEC.
 - Your [CAA records](/ssl/edge-certificates/caa-records/) should allow Cloudflare's partner [certificate authorities (CAs)](/ssl/reference/certificate-authorities/) to issue certificates on your behalf.
 - The HTTP verification process is done preferably over **IPv6**, so if any `AAAA` record exists and does not point to the same dual-stack location as the `A` record, the validation will fail.
+- If a NS is present for the hostname or its parent, DNS resolution will be managed externally by the DNS provider defined in the NS target. In this case the DCV TXT record needs to be added at the external DNS provider, or the NS needs to be removed.
 
 ## CA errors