cluster-management: Clarify the use of cloud-init with networkd units

Expanding it just a bit to tell you how and when to reload networkd:
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Drop a networkd unit in `/etc/systemd/network/` or inject a unit on boot via cloud-config to override an existing unit. Network units injected in cloud-config will automatically trigger a networkd reload in order for changes to be applied. Files placed on the filesystem will need to reload networkd afterwards with `sudo systemctl restart systemd-networkd`

Let's take a look at two common situations: using a static IP and turning off DHCP.