runtime: Clarify cgroup cleanup on 'delete'

Expanding on the creater-owns-the-resource cases from the previous
commit.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

runtime.md

@@ -129,6 +129,7 @@ Resources associated with the container, but not created by this container, MUST
 On Linux, [`mounts`](config.md#mounts), [`root`](config.md#root), [`linux.devices`](config-linux.md#devices), and other filesystem changes belong to the container which created the [container mount namespace][container-namespace3].
 If a container joins an existing mount namespace and applies those settings, deleting the container MUST NOT reverted the settings.
 Similarly [`hostname`](config.md#hostname) changes belong to the container which created the container UTS namespace.
+[`linux.resources`](config-linux.md#control-groups) settings belong to the container which created the associated cgroup.
 
 ## Hooks
 Many of the operations specified in this specification have "hooks" that allow for additional actions to be taken before or after each operation.