Show from and to address when detecting a loop they may aid in
debugging.

Hard to create a unit test, but this is a startup run with self induced
loop:

~~~ corefile
.:1053 {
    loop
    log
    forward . 127.0.0.1:1053
}
~~~~

~~~
:1053
2018-12-16T10:11:03.695Z [INFO] CoreDNS-1.3.0
2018-12-16T10:11:03.695Z [INFO] linux/amd64, go1.11,
CoreDNS-1.3.0
linux/amd64, go1.11,
2018-12-16T10:11:03.696Z [FATAL] plugin/loop: Loop (127.0.0.1:51384 -> :1053) detected for zone ".", see https://coredns.io/plugins/loop#troubleshooting. Query: "HINFO 2781022615773629442.4133547885299871809."
~~~

Update the docs and polished that a bit as well.
Signed-off-by: Miek Gieben <miek@miek.nl>