samples: net: syslog: Add info how to manually start net backend

If CONFIG_LOG_BACKEND_NET_AUTOSTART is disabled, then the syslog
network backend is not started automatically. This option is
enabled by default but it might be that application needs to
start the backend manually if the network is slow to start or
some special routes needs to be set by the application.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>