cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst

Update Documentation/admin-guide/cgroup-v2.rst on the newly introduced
"isolated" cpuset partition type as well as other changes made in other
cpuset patches.

Signed-off-by: Waiman Long <longman@redhat.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>