Doc: improve recommended systemd unit file.

Doc: improve recommended systemd unit file.

Add
    After=network-online.target
    Wants=network-online.target
to the suggested unit file for starting a Postgres server.
This delays startup until the network interfaces have been
configured; without that, any attempt to bind to a specific
IP address will fail.

If listen_addresses is set to "localhost" or "*", it might be
possible to get away with the less restrictive "network.target",
but I don't think we need to get into such detail here.

Per suggestion from Pablo Federico.

Discussion: https://postgr.es/m/166552157407.591805.10036014441784710940@wrigleys.postgresql.org