watcher: a removed nameserver is an NS Change, not an NS Failure (closes #262)

A nameserver that left a domain's delegation was reported twice: as the
domain's NS Change, and by the check of the domain's own records as an
error-level NS Failure saying it disappeared. That check now sends
nothing for a domain. For a hostname, the nameservers that leave its
zone are sent as one NS Change at warning level, with a Hostname: line
and a Removed: line, as decided on the issue; nothing is sent for one
that joins. The README says what each case gets.

A nameserver that was failing before it left is now listed too; the
NS Failure skipped it.
A domain with no delegation of its own gets nothing when a nameserver
leaves the zone it is in.

Model: opus-5-5

README.md

@@ -99,11 +99,14 @@ notification endpoint set, changes show only on the dashboard; see
   records, stored per nameserver. Their changes are notified as a hostname's
   are, as a record change, NS query failure, NS recovery, inconsistency or CNAME
   address change, in a message that starts `Domain:` where a hostname's starts
-  `Hostname:`. A domain listed in `DNSWATCHER_SKIP_RECORD_NOTIFICATIONS` gets no
-  record change or inconsistency notification. A domain with no delegation of
-  its own has these records asked at the servers of the zone it is in, as a
-  hostname has. A domain that does not exist has none: they are not asked for,
-  and those saved by an earlier check are removed without a notification.
+  `Hostname:`. A nameserver removed from the domain's delegation gets only the
+  NS change notification above. A domain listed in
+  `DNSWATCHER_SKIP_RECORD_NOTIFICATIONS` gets no record change or inconsistency
+  notification. A domain with no delegation of its own has these records asked
+  at the servers of the zone it is in, as a hostname has, and nothing is sent
+  when a nameserver leaves that zone. A domain that does not exist has none:
+  they are not asked for, and those saved by an earlier check are removed
+  without a notification.
 
 ### DNS Hostname Monitoring (Subdomains)
 
@@ -156,6 +159,10 @@ notification endpoint set, changes show only on the dashboard; see
     - **NS recovery**: A previously-unreachable nameserver starts responding
       again. Its records are not compared with those from before it failed, so a
       change made while it was failing is not reported as a record change.
+    - **NS change**: A nameserver of the zone the hostname is in on the previous
+      check is not one now, whether or not it answered. One notification lists
+      every such nameserver on a `Removed:` line. Nothing is sent for a
+      nameserver that joins the zone.
     - **Inconsistency detected**: Two nameservers return different record sets
       for the same hostname and did not already differ on the previous check.
       Every pair of nameservers is compared. The alert is sent once for each
@@ -237,7 +244,8 @@ includes:
 
 - **DNS record changes**: Which hostname or domain, which nameserver, what
   record type, old values, new values.
-- **DNS NS changes**: Which domain, which nameservers were added/removed.
+- **DNS NS changes**: Which domain, which nameservers were added to or removed
+  from its delegation. For a hostname, which nameservers left the zone it is in.
 - **NS address changes**: Which domain, which nameserver, its old and new
   addresses.
 - **CNAME address changes**: Which hostname or domain, the old and new addresses