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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/cgroup-v1/memory.rst

@@ -851,6 +851,9 @@ At reading, current status of OOM is shown.
 	  (if 1, oom-killer is disabled)
 	- under_oom	   0 or 1
 	  (if 1, the memory cgroup is under OOM, tasks may be stopped.)
+        - oom_kill         integer counter
+          The number of processes belonging to this cgroup killed by any
+          kind of OOM killer.
 
 11. Memory Pressure
 ===================