config-linux: clarify cgroupsPath

Clarify some of the confusion with cgroupsPath. Due to systemd, we
cannot require that relative paths be treated in any specific way. In
addition, add a line stating that not all values of cgroupsPath are
required to be valid (and that runtimes must error out if they have an
invalid cgroup path).

Signed-off-by: Aleksa Sarai <asarai@suse.de>

Author: cyphar

config-linux.md

@@ -158,8 +158,12 @@ cgroups provide controls to restrict cpu, memory, IO, pids and network for the c
 For more information, see the [kernel cgroups documentation][cgroup-v1].
 
 The path to the cgroups can be specified in the Spec via `cgroupsPath`.
-`cgroupsPath` is expected to be relative to the cgroups mount point.
-If `cgroupsPath` is not specified, implementations can define the default cgroup path.
+If `cgroupsPath` is
+* ... an absolute path (starting with `/`), the implementation MUST take the path to be relative to the cgroup mount point.
+* ... a relative path (not starting with `/`), implementations SHOULD take the value to be the suffix of the resulting cgroup path.
+* ... not specified, implementations CAN define the default cgroup path.
+Implementations CAN consider certain `cgroupsPath` values to be invalid, and MUST generate an error if this is the case.
+
 Implementations of the Spec can choose to name cgroups in any manner.
 The Spec does not include naming schema for cgroups.
 The Spec does not support [split hierarchy][cgroup-v2].