The memcg code does not allow changing memory.use_hierarchy if the
parent cgroup has enabled use_hierarchy.  Update documentation to match
the code.
Signed-off-by: NGreg Thelen <gthelen@google.com>
Acked-by: NKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>