POLA: principle of least astonishement. for an admin not familiar with
the blackbox exporter, that `curl` is a huge WTF. so we instead show
the more familiar nc command, and also explain what it does.

*then* we introduce the blackbox stuff.

service/donate.md

@@ -410,14 +410,18 @@ strongSwan. See also the [IPsec documentation](howto/ipsec) for more
 troubleshooting tricks.
 
 If the tunnel is up, you can check that you can reach the service from the
-frontend server:
+frontend server. Redis uses a simple text-based protocol over TCP, and
+there's a PING command you can use to test availability:
 
-    curl 'http://localhost:9115/probe?target=crm-int-01-priv:6379&module=redis_banner&debug=true'
-    # The following is equivalent but with less debugging info:
     echo PING | nc -w 1 crm-int-01-priv 6379
 
-If you can't reach the service, check on the crm-int-01.tpo server that the
-redis service is correctly running.
+Or you can try reproducing the blackbox probe directly, with:
+
+    curl 'http://localhost:9115/probe?target=crm-int-01-priv:6379&module=redis_banner&debug=true'
+
+If you can't reach the service, check on the [CiviCRM server](howto/civicrm)
+(currently `crm-int-01.torproject.org`) that the Redis service is
+correctly running.
 
 ## Disaster recovery