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). However, any given value of cgroupsPath should
provide consistent results.

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

config-linux.md

@@ -159,8 +159,13 @@ For more information, see the [kernel cgroups documentation][cgroup-v1].
 
 The path to the cgroups can be specified in the Spec via `cgroupsPath`.
 `cgroupsPath` can be used to either control the cgroup hierarchy for containers or to run a new process in an existing container.
-`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 runtime MUST take the path to be relative to the cgroup mount point.
+* ... a relative path (not starting with `/`), the runtime MAY interpret the path relative to a runtime-determined location in the cgroup hierarchy.
+* ... not specified, the runtime MAY define the default cgroup path.
+Runtimes MAY consider certain `cgroupsPath` values to be invalid, and MUST generate an error if this is the case.
+If a `cgroupsPath` value is specified, the runtime MUST consistently attach to the same place in the cgroup hierarchy given the same value of `cgroupsPath`.
+
 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 per-controller paths for the reasons discussed in the [cgroupv2 documentation][cgroup-v2].