docs: vm-external-vlan states the vm-instance chart has no networkData, which cozystack#4306 changes #702

content/en/docs/next/networking/vm-external-vlan.md and the v1.6 copy both state that the vm-instance chart has no networkData and wires cloud-init through userData only. cozystack/cozystack#4306 changes that: it adds a netplanDhcp value, off by default, which makes the chart supply its own cloud-init network-data asking every en* and eth* interface for DHCP and accepting the DNS search domains offered over DHCP.

Two paragraphs on each page go stale when that PR lands. The first is in Step 3 — Attach the VM and assign a static address, which explains the userData recipe by the chart not supporting networkData; the reason changes, the recipe does not, and the VM in the recipe keeps netplanDhcp off. The second is the No networkData bullet under Gotchas, which becomes a warning to leave netplanDhcp off on a VM with a static NIC: netplan writes one networkd file per definition ID, systemd-networkd applies only the first matching .network file in lexical order, and the chart's 10-netplan-cozy-en.network sorts before the recipe's 10-netplan-enp2s0.network, so the static address would never be applied.

The equivalent paragraphs in this repository's own docs/vm-external-vlan.md are updated in that PR, so the wording can be taken from there once it merges. Filing this rather than a PR because the pages are per-version and the change should not land here before it lands upstream.