[DNS] Update account-custom-nameservers.mdx

Author: ngayerie

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

@@ -26,6 +26,13 @@ For this configuration to be possible, a few conditions apply:
 
 * Choosing a set from `ns_set 1` through `ns_set 5` will influence how Cloudflare assigns nameservers to your new zones if you configure [DNS zone defaults](/dns/nameservers/nameserver-options/#dns-zone-defaults).
 
+:::caution
+
+* Zone custom nameservers are available for zones on Business or Enterprise plans. Via API or on the dashboard.
+* 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.
+
+:::
+
 ## Enable account custom nameservers
 
 ### 1. Set up ACNS names and sets
@@ -167,4 +174,4 @@ 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>