Merge branch 'systemctl-help' into 'master'

add Documentation= reference to knot-resolver.service

This makes "systemctl help knot-resolver" bring up the appropriate man
page.

See merge request !39

systemd/knot-resolver.service

 [Unit]
 Description=Knot DNS Resolver daemon
+Documentation=man:kresd(8)
 ## This is a socket-activated service:
 RefuseManualStart=true