undocumented value of dhcp-identifier

[Waldek M.]

Current documentation of netplan states the following:

https://github.com/CanonicalLtd/netplan/blob/master/doc/netplan.md

    dhcp-identifier (scalar)

    : When set to 'mac'; pass that setting over to systemd-networkd to use the device's MAC address as a unique identifier rather than a RFC4361-compliant Client ID. This has no effect when NetworkManager is used as a renderer.

It doesn't however list other possible values, including the default one.
After reading the source code at https://github.com/CanonicalLtd/netplan/blob/master/src/parse.c#L1388 I found out it's 'duid'.

In my opinion the values should be documented, especially the default.