Registrar: only for zones that are a registered domain

With porkbun.domains empty the Registrar tab was offered for every zone
and always failed for subdomain zones, reverse zones and private names.
dnsname.RegisteredDomain (public suffix list) now decides; the others
get a message naming what delegates them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/dnssec.md

@@ -204,6 +204,18 @@ root:jarl`, `chmod 640`, as the package installs it); jarld warns at start
 when everyone can read it. Only administrators can use the registrar
 features.
 
+Only a zone that is a registered domain gets the registrar features
+(`adbraun.lol`, `example.co.uk`, judged by the public suffix list). Their
+parent zone delegates the others, not a registrar:
+
+* a subdomain zone such as `lab.adbraun.lol`: the NS and DS records in
+  `adbraun.lol`;
+* a reverse zone: whoever holds the addresses (your ISP, or the RIR);
+* a zone under a suffix that isn't public (`home.lan`, `home.arpa`):
+  nobody.
+
+The zone's **Name servers** tab (Check delegation) checks all of them.
+
 **Using it**: the zone's **Registrar** tab, or `jarl zone registrar`:
 
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