Add to docs: verify DNS query goes through unbound

[sed-i]

After configuring pihole to use unbound,

docs/docs/guides/dns/unbound.md

Line 180 in a7342b8

Finally, configure Pi-hole to use your recursive DNS server by specifying `127.0.0.1#5335` as the Custom DNS (IPv4):

it would be handy if the docs included a method to verify that a DNS query is indeed going through unbound.

[yubiuser]

Pretty simple

cat /var/log/pihole.log

Apr 27 22:07:00 dnsmasq[32267]: query[A] play.google.com from 10.0.1.41
Apr 27 22:07:00 dnsmasq[32267]: forwarded play.google.com to 127.0.0.1#5335
Apr 27 22:07:00 dnsmasq[32267]: reply play.google.com is 142.251.37.14