doc: memcontrol: add description for oom_kill

doc: memcontrol: add description for oom_kill

When debugging an oom issue, I found the oom_kill counter of memcg is
confusing.  At the first glance without checking document, I thought it
just counts for memcg oom, but it turns out it counts both global and
memcg oom.

The cgroup v2 documents it, but the description is missed for cgroup v1.

Signed-off-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Chris Down <chris@chrisdown.name>
Link: https://lore.kernel.org/r/20210226021254.3980-1-shy828301@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>