[DNS] Explicitly state availability in custom NS how-tos (#19080)

* [DNS] Update account-custom-nameservers.mdx

* Add availability h2 to all custom NSs procedure pages

---------

Co-authored-by: Rebecca Tamachiro <[email protected]>

Author: ngayerie

src/content/docs/dns/nameservers/custom-nameservers/account-custom-nameservers.mdx

@@ -16,6 +16,10 @@ import { Example, Render, Tabs, TabItem } from "~/components"
 
 <Render file="acns-tcns-intro" params={{ one: "Account", two: "A", three: "account", four: "zones", five: "account " }} />
 
+## Availability
+
+Account custom nameservers are available for customers on Business (after [contacting Cloudflare Support](/support/contacting-cloudflare-support/)) or Enterprise plans. Once configured, account custom nameservers can be used by all zones in the account, regardless of the zone plan. Via API or on the dashboard.
+
 ## Configuration conditions
 
 For this configuration to be possible, a few conditions apply:
@@ -167,4 +171,4 @@ Following the [configuration conditions](#configuration-conditions), each set mu
 Use the [Delete account custom nameserver endpoint](/api/resources/custom_nameservers/methods/delete/) to delete a specific ACNS.
 
 </TabItem>
-</Tabs>
+</Tabs>

src/content/docs/dns/nameservers/custom-nameservers/tenant-custom-nameservers.mdx

@@ -18,6 +18,10 @@ import { Example, Render } from "~/components"
 
 <Render file="acns-tcns-intro" params={{ one: "Tenant", two: "T", three: "tenant", four: "accounts", five: "tenant " }} />
 
+## Availability
+
+Tenant custom nameservers, if created by the tenant owner, will be available to all zones belonging to any account that is part of the tenant. Via API only.
+
 ## Configuration conditions
 
 For this configuration to be possible, a few conditions apply:

src/content/docs/dns/nameservers/custom-nameservers/zone-custom-nameservers.mdx

@@ -18,6 +18,10 @@ With zone custom nameservers (ZCNS), each custom nameserver name must be a subdo
 
 For example, for a zone `domain.test`, the ZCNS can be `ns1.domain.test` and `ns2.domain.test` but they cannot use a different TLD (`ns1.domain.org`) nor a different domain (`ns1.example.com`).
 
+## Availability
+
+Zone custom nameservers are available for zones on Business or Enterprise plans. Via API or on the dashboard.
+
 ## Use zone custom nameservers
 
 ### Primary (full setup) zones