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>

Documentation/cgroups/memory.txt

@@ -485,8 +485,9 @@ The feature can be disabled by
 
 # echo 0 > memory.use_hierarchy
 
-NOTE1: Enabling/disabling will fail if the cgroup already has other
-       cgroups created below it.
+NOTE1: Enabling/disabling will fail if either the cgroup already has other
+       cgroups created below it, or if the parent cgroup has use_hierarchy
+       enabled.
 
 NOTE2: When panic_on_oom is set to "2", the whole system will panic in
        case of an OOM event in any cgroup.