cgroups.7: Add a detail on delegation of cgroup.threads

cgroups.7: Add a detail on delegation of cgroup.threads

Some notes from a conversation with Tejun Heo:

    Subject: Re: cgroups(7): documenting cgroups v2 delegation
    Date: Wed, 10 Jan 2018 14:27:26 -0800
    From: Tejun Heo <tj@kernel.org>

    > > 1. When delegating, cgroup.threads should be delegated.  Doing that
    > >    selectively doesn't achieve anything meaningful.
    >
    > Understood. But surely delegating cgroup.threads is effectively
    > meaningless when delegating a "domain" cgroup tree? (Obviously it's
    > not harmful to delegate the the cgroup.threads file in this case;
    > it's just not useful to do so.)

    Yeap, unless we can somehow support non-root mixed domains.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>