Add Documentation fields to systemd service files

We point to the manpages where available and redirect to libvirt's
homepage as a last resort.

(cherry picked from commit 1b9f5aa7)

daemon/libvirtd.service.in

@@ -9,6 +9,8 @@ Before=libvirt-guests.service
 After=network.target
 After=dbus.service
 After=iscsid.service
+Documentation=man:libvirtd(8)
+Documentation=http://libvirt.org
 
 [Service]
 EnvironmentFile=-/etc/sysconfig/libvirtd

src/locking/virtlockd.service.in

 [Unit]
 Description=Virtual machine lock manager
 Requires=virtlockd.socket
+Documentation=man:virtlockd(8)
+Documentation=http://libvirt.org
 
 [Service]
 EnvironmentFile=-/etc/sysconfig/virtlockd

tools/libvirt-guests.service.in

 [Unit]
 Description=Suspend Active Libvirt Guests
 After=network.target libvirtd.service
+Documentation=man:libvirtd(8)
+Documentation=http://libvirt.org
 
 [Service]
 EnvironmentFile=-/etc/sysconfig/libvirt-guests