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docs/WRITING VM_AND_CONTAINER_MANAGERS: update manpage link

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docs/WRITING_VM_AND_CONTAINER_MANAGERS.md
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@ -27,28 +27,12 @@ their own.
## Host OS Integration
All virtual machines and containers should be registered with the
[systemd-machined(8)](https://www.freedesktop.org/software/systemd/man/latest/systemd-machined.service.html)
mini service that is part of systemd. This provides integration into the core
OS at various points. For example, tools like ps, cgls, gnome-system-manager
use this registration information to show machine information for running
processes, as each of the VM's/container's processes can reliably attributed to
a registered machine. The various systemd tools (like systemctl, journalctl,
loginctl, systemd-run, ...) all support a -M switch that operates on machines
registered with machined. "machinectl" may be used to execute operations on any
such machine. When a machine is registered via machined its processes will
automatically be placed in a systemd scope unit (that is located in the
machines.slice slice) and thus appear in "systemctl" and similar commands. The
scope unit name is based on the machine meta information passed to machined at
registration.
All virtual machines and containers should be registered with the [machined](https://www.freedesktop.org/software/systemd/man/latest/org.freedesktop.machine1) mini service that is part of systemd. This provides integration into the core OS at various points. For example, tools like ps, cgls, gnome-system-manager use this registration information to show machine information for running processes, as each of the VM's/container's processes can reliably attributed to a registered machine. The various systemd tools (like systemctl, journalctl, loginctl, systemd-run, ...) all support a -M switch that operates on machines registered with machined. "machinectl" may be used to execute operations on any such machine. When a machine is registered via machined its processes will automatically be placed in a systemd scope unit (that is located in the machines.slice slice) and thus appear in "systemctl" and similar commands. The scope unit name is based on the machine meta information passed to machined at registration.
For more details on the APIs provided by machine consult [the bus API interface
documentation](https://www.freedesktop.org/software/systemd/man/latest/org.freedesktop.machine1.html).
For more details on the APIs provided by machine consult [the bus API interface documentation](https://www.freedesktop.org/software/systemd/man/latest/org.freedesktop.machine1).
## Guest OS Integration
A number of interfaces are defined that permit a machine or container manager
to set provide integration points with the payload/guest system. These
interfaces are documented in [Container Interface of
systemd](https://systemd.io/CONTAINER_INTERFACE) and [VM Interface of
systemd](https://systemd.io/VM_INTERFACE).
As container virtualization is much less comprehensive, and the guest is less isolated from the host, there are a number of interfaces defined how the container manager can set up the environment for systemd running inside a container. These Interfaces are documented in [Container Interface of systemd](CONTAINER_INTERFACE).
VM virtualization is more comprehensive and fewer integration APIs are available. In fact there's only one: a VM manager may initialize the SMBIOS DMI field "Product UUUID" to a UUID uniquely identifying this virtual machine instance. This is read in the guest via /sys/class/dmi/id/product_uuid, and used as configuration source for /etc/machine-id if in the guest, if that file is not initialized yet. Note that this is currently only supported for kvm hosts, but may be extended to other managers as well.