This was dropped on reviewers' request in the revision that got merged,
but reference in two documents was not updated. Fix it.
Follow-up for: https://github.com/systemd/systemd/pull/25918
This is intended to be used with VSOCK, to notify the hypervisor/VMM, eg on the host:
qemu <...> -smbios type=11,value=io.systemd.credential:vmm.notify_socket=vsock:2:1234 -device vhost-vsock-pci,id=vhost-vsock-pci0,guest-cid=42
(vsock:2:1234 -> send to host on vsock port 1234, default is to send to 0 which is
the hypervisor itself)
Also on the host:
$ socat - VSOCK-LISTEN:1234,socktype=5
READY=1
STATUS=Ready.
Note that this drops ProtectProc=invisible from
systemd-resolved.service.
This is done because othewise access to the booted "kernel" command line is not
necessarily available. That's because in containers we want to read
/proc/1/cmdline for that.
Fixes: #24103
