config-linux: allow lazy cgroup handling

Make explicit that runtimes only have to attach to the bare minimum
number of cgroups in order to fulfil the users' requirements. However,
runtimes are of course allowed to attach to more than the bare minimum.

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

config-linux.md

@@ -169,6 +169,9 @@ The Spec does not include naming schema for cgroups.
 The Spec does not support [split hierarchy][cgroup-v2].
 The cgroups will be created if they don't exist.
 
+A runtime MUST at least use the minimum set of cgroups required to fulfil the resource restrictions specified by `config.json`.
+However, a runtime CAN decide to not attach to all cgroups supported by the system.
+
 ###### Example
 
 ```json