cgroup: add documentation for the PIDs controller

Add documentation derived from kernel/cgroup_pids.c to the relevant
Documentation/ directory, along with a few examples of how to use the
PIDs controller as well an explanation of its peculiarities.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Tejun Heo <tj@kernel.org>