[DNS] Clarify that assigned nameservers cannot be changed (#33037)

* Clarify that assigned nameservers cannot be changed

DEE-3776

Common customer question: 'why did my new zone get different
nameservers than my other zones, can we reassign it?' The answer is
already documented but easy to miss. Small changes to make it more
discoverable:

- DNS FAQ: new entry 'Why does my new zone have different nameservers
  than my other zones?' explaining the reasons a new zone can be
  assigned a different pair (cross-account conflict, prior deletion,
  parent/child rules, Foundation DNS color rotation) and how to get
  consistent nameservers via Account custom nameservers or DNS zone
  defaults.
- Nameserver options: replace a plain sentence with a callout that
  points to the new FAQ entry and to the Nameserver consistency section.
- Foundation DNS advanced nameservers: new subsection under Nameservers
  hosting and assignment that explains why a new zone might get a
  different color set (blue / gold / orange) and cross-links Account
  custom nameservers.

* Style guide: remove directional word from Nameserver consistency link

* Address review feedback from @hannes-cf

- faq.mdx: reword 'a different pair' -> 'different nameservers'; drop
  'pair' in the parent/child bullet; use 'set' consistently in the
  delete-and-re-add caveat.
- foundation-dns/advanced-nameservers.mdx: 'The same name is' ->
  'The same domain is', matching the FAQ wording.
- nameservers/nameserver-options.mdx: replace titled ':::note[...]' with
  an untitled ':::caution' (per docs convention and given severity); in
  the adjacent pre-existing caution, replace 'a new pair of nameservers'
  with 'a new set of nameservers' for wording consistency.

Also rebased onto origin/production.

Author: ngayerie

src/content/docs/dns/faq.mdx

@@ -92,6 +92,24 @@ This is important because, if a domain is in a **Moved** state for a [long enoug
 
 <Render file="recover-deleted-domain" product="dns" />
 
+### Why does my new zone have different nameservers than my other zones?
+
+Nameserver assignments happen at zone creation and [cannot be changed](/dns/nameservers/nameserver-options/#assignment-method) afterwards, even by Cloudflare Support.
+
+A newly added zone can be assigned different nameservers from your other zones for a few reasons:
+
+- The same domain is (or was recently) active on another Cloudflare account.
+- The zone was previously deleted from Cloudflare and re-added.
+- A parent or child zone in the same account already uses the preferred nameservers.
+- The account uses [Foundation DNS advanced nameservers](/dns/foundation-dns/advanced-nameservers/), which use different sets (`blue`, `gold`, `orange`) and rotate to keep [directly descending zones](/dns/foundation-dns/advanced-nameservers/#nameservers-hosting-and-assignment) on different nameservers.
+
+To make future zones share the same nameservers, use one of the following options depending on your plan:
+
+- [Account custom nameservers](/dns/nameservers/custom-nameservers/account-custom-nameservers/) — available on Enterprise (self-serve), or on Business after [contacting Cloudflare Support](/support/contacting-cloudflare-support/) to enable them.
+- [DNS zone defaults](/dns/additional-options/dns-zone-defaults/) with advanced nameservers or account custom nameservers — available on Enterprise.
+
+Deleting and re-adding the zone does not force a specific nameserver assignment and can produce yet another different set.
+
 ---
 
 ## DNS records

src/content/docs/dns/foundation-dns/advanced-nameservers.mdx

@@ -74,6 +74,16 @@ Consider the domain `example.com`, and subdomains `abc.example.com` and `123.exa
 
 </Details>
 
+### Consistent assignment across new zones
+
+Advanced nameservers try to keep the same nameserver set (`blue`, `gold`, or `orange`) for new zones added to the same account, but a new zone can still be assigned a different set when:
+
+- The same domain is (or was recently) active on another Cloudflare account.
+- A directly descending zone in the same or another account already uses the same set.
+- The zone was previously deleted from Cloudflare and re-added.
+
+[Assigned nameservers cannot be changed](/dns/nameservers/nameserver-options/#assignment-method) after a zone is created. If your zones must share the same nameservers, [account custom nameservers](/dns/nameservers/custom-nameservers/account-custom-nameservers/) provide a single set that every zone in the account can use, and can be configured as the account's [DNS zone default](/dns/additional-options/dns-zone-defaults/) so new zones automatically receive them.
+
 ## Custom Nameserver compatibility
 
 Advanced Nameserver features — such as multiple anycast network groups or dedicated release process — are currently available for Cloudflare-branded nameservers. Support of these features for [Custom Nameservers](/dns/nameservers/custom-nameservers/) is on the roadmap. Contact your account team for the latest availability.

src/content/docs/dns/nameservers/nameserver-options.mdx

@@ -22,10 +22,12 @@ When you add a domain on a [primary (full)](/dns/zone-setups/full-setup/) or [se
 The default assignment method is to use [standard nameservers](/dns/nameservers/#standard-nameservers) and favor consistent nameserver names across all zones within an account. Nonetheless, in case there are conflicts, you may get different nameserver names, even for domains that are within the same account.
 
 :::caution
-To prevent domain hijacking, you can no longer preset Cloudflare nameservers at your registrar before creating the respective zone in Cloudflare. If you preset your nameservers and then add the domain, your domain will be assigned a new pair of nameservers.
+To prevent domain hijacking, you can no longer preset Cloudflare nameservers at your registrar before creating the respective zone in Cloudflare. If you preset your nameservers and then add the domain, your domain will be assigned a new set of nameservers.
 :::
 
-These nameserver assignments cannot be changed. However, depending on your subscription, you may have different options for better nameserver consistency.
+:::caution
+Once a zone is created, its assigned nameservers cannot be changed — not even by Cloudflare Support. If your new zone was assigned a different set from your other zones, refer to [Why does my new zone have different nameservers than my other zones?](/dns/faq/#why-does-my-new-zone-have-different-nameservers-than-my-other-zones) in the DNS FAQ. To keep new zones on the same nameservers going forward, refer to [Nameserver consistency](#nameserver-consistency).
+:::
 
 ### Nameserver consistency