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>