docs, cgroup: add entries for pids to cgroup-v2.rst

This patch add two entries (pids.peak and pids.events) for pids
controller, and also update pids.current because it's on non-root.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: Xiu Jianfeng

Documentation/admin-guide/cgroup-v2.rst

@@ -2184,11 +2184,25 @@ PID Interface Files
 	Hard limit of number of processes.
 
   pids.current
-	A read-only single value file which exists on all cgroups.
+	A read-only single value file which exists on non-root cgroups.
 
 	The number of processes currently in the cgroup and its
 	descendants.
 
+  pids.peak
+	A read-only single value file which exists on non-root cgroups.
+
+	The maximum value that the number of processes in the cgroup and its
+	descendants has ever reached.
+
+  pids.events
+	A read-only flat-keyed file which exists on non-root cgroups. The
+	following entries are defined. Unless specified otherwise, a value
+	change in this file generates a file modified event.
+
+	  max
+		Number of times fork failed because limit was hit.
+
 Organisational operations are not blocked by cgroup policies, so it is
 possible to have pids.current > pids.max.  This can be done by either
 setting the limit to be smaller than pids.current, or attaching enough