cgroup/cpuset: Remove stale text

Task's cpuset pointer was removed by
commit 8793d854 ("Task Control Groups: make cpusets a client of cgroups")

Paragraph "The task_lock() exception ...." was removed by
commit 2df167a3 ("cgroups: update comments in cpuset.c")

Remove stale text:

 We also require taking task_lock() when dereferencing a
 task's cpuset pointer. See "The task_lock() exception", at the end of this
 comment.

 Accessing a task's cpuset should be done in accordance with the
 guidelines for accessing subsystem state in kernel/cgroup.c

and reformat.

Co-developed-by: Michal Koutný <mkoutny@suse.com>
Co-developed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>