memcg: clarify use_hierarchy documentation

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: Greg Thelen <gthelen@google.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>