clone.2: Note a performance benefit of CLONE_INTO_CGROUP

clone.2: Note a performance benefit of CLONE_INTO_CGROUP

As noted in email by Christian Brauner:

    I forgot to mention that spawning directly into a target
    cgroup is also more efficient than moving it after creation.
    The specific reason is mentioned in the commit message
    [ef2c41cf38a], the write lock of the semaphore need not be
    taken in contrast to when it is moved afterwards. That
    implementation details is not that interesting but it might
    be interesting to know that it provides performance benefits
    in general.

Reported-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>