[DNS] Update cname-domain-verification.mdx

Author: ngayerie

src/content/docs/dns/manage-dns-records/troubleshooting/cname-domain-verification.mdx

@@ -20,11 +20,15 @@ You may find issues if you have one of the following:
 * The CNAME record you created for domain verification is set to [**Proxied**](/dns/proxy-status/).
 * The CNAME record is correctly set to DNS only (not proxied) but, in your [zone settings](https://dash.cloudflare.com/?to=/:account/:zone/dns/settings), [**CNAME flattening for all CNAME records**](/dns/cname-flattening/set-up-cname-flattening/#for-all-cname-records) is enabled.
 * The CNAME record is correctly set to DNS only (not proxied) but CNAME flattening is [enabled for that record specifically](/dns/cname-flattening/set-up-cname-flattening/#per-record).
+* A [NS record](https://www.cloudflare.com/learning/dns/dns-records/dns-ns-record/) exists for the subdomain
 
 ## Solution
 
 Make sure that:
 
 * In your zone DNS settings: [**CNAME flattening for all CNAME records**](/dns/cname-flattening/) is disabled.
 * On the DNS records table: you have filled in the CNAME record fields correctly, proxy status is set to **DNS only**, and **Flatten** is disabled.
+* Review the NS configuration, and either:
+  * make sure that your record is configured as expected with the service provider that the NS record points towards
+  * remove the NS record so that the DNS resolution this record is resolved to the target you've configured on Cloudflare. (but be careful as this may affect other records!)