Document the resolv.conf handling for the default host-local IPAM

[radekg]

The user can specify a bridge specific resolv.conf file with custom nameservers configured: https://www.cni.dev/plugins/current/ipam/host-local/. Example:

{
  "ipam": {
		"type": "host-local",
		"subnet": "3ffe:ffff:0:01ff::/64",
		"rangeStart": "3ffe:ffff:0:01ff::0010",
		"rangeEnd": "3ffe:ffff:0:01ff::0020",
		"routes": [
			{ "dst": "3ffe:ffff:0:01ff::1/64" }
		],
		"resolvConf": "/etc/resolv.conf"
	}
}

The contents of this resolv.conf are passed to the VM so it is not necessary to handle resolv.conf in vminit. Document.