mm: memcontrol: reword obsolete comment of mem_cgroup_unmark_under_oom()

mm: memcontrol: reword obsolete comment of mem_cgroup_unmark_under_oom()

Since commit 79dfdaccd1d5 ("memcg: make oom_lock 0 and 1 based rather than
counter"), the mem_cgroup_unmark_under_oom() is added and the comment of
the mem_cgroup_oom_unlock() is moved here.  But this comment make no sense
here because mem_cgroup_oom_lock() does not operate on under_oom field.
So we reword the comment as this would be helpful.  [Thanks Michal Hocko
for rewording this comment.]

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Link: https://lkml.kernel.org/r/20200930095336.21323-1-linmiaohe@huawei.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>