memcg: document memory.low_limit_in_bytes

memcg: document memory.low_limit_in_bytes

Describe low_limit_in_bytes and its effect.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Roman Gushchin <klamm@yandex-team.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>