cgroup/cpuset: Improve cpuset_css_alloc() description

Change the function argument in the description of cpuset_css_alloc()
from 'struct cgroup' -> 'struct cgroup_subsys_state'.  The change to the
argument type was introduced by commit eb95419b ("cgroup: pass
around cgroup_subsys_state instead of cgroup in subsystem methods").
Also, add more information to its description.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Acked-by: Waiman Long <longman@redhat.com>
Acked-by: Joel Savitz <jsavitz@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>