doc/networks: describe how to notify systemd-resolved of lxd nameserver
doc/networks: describe how to notify systemd-resolved of lxd nameserver

Signed-off-by: Dan Streetman <9cf7b61c8faa5cbb1bbe4dcb6838d42eed5e6946@canonical.com>

hallyn/lxd,Skarlso/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,atxwebs/lxd,Skarlso/lxd,Skarlso/lxd,lxc/lxd,lxc/lxd,atxwebs/lxd,stgraber/lxd,lxc/lxd,atxwebs/lxd,mjeanson/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,mjeanson/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,Skarlso/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,mjeanson/lxd,hallyn/lxd,hallyn/lxd