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>

kernel/cgroup/cpuset.c

@@ -3072,11 +3072,15 @@ static struct cftype dfl_files[] = {
 };
 
 
-/*
- *	cpuset_css_alloc - allocate a cpuset css
- *	cgrp:	control group that the new cpuset will be part of
+/**
+ * cpuset_css_alloc - Allocate a cpuset css
+ * @parent_css: Parent css of the control group that the new cpuset will be
+ *              part of
+ * Return: cpuset css on success, -ENOMEM on failure.
+ *
+ * Allocate and initialize a new cpuset css, for non-NULL @parent_css, return
+ * top cpuset css otherwise.
  */
-
 static struct cgroup_subsys_state *
 cpuset_css_alloc(struct cgroup_subsys_state *parent_css)
 {