cgroup-legacy may be too loaded.  Rename the docs so that they're
postfixed with v1 and v2.

* s/cgroup-legacy/cgroup-v1/
* s/cgroup.txt/cgroup-v2.txt/
Signed-off-by: NTejun Heo <tj@kernel.org>

In preparation for adding cgroup2 documentation, rename
Documentation/cgroups/ to Documentation/cgroup-legacy/.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

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