kernel-doc: fix sched.c missing parameter

Add missing kernel-doc in kernel/sched.c:

Warning(linux-2.6.25-git3//kernel/sched.c:7044): No description found for parameter 'span'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sched.c

@@ -7035,6 +7035,7 @@ static int find_next_best_node(int node, nodemask_t *used_nodes)
 /**
  * sched_domain_node_span - get a cpumask for a node's sched_domain
  * @node: node whose cpumask we're constructing
+ * @span: resulting cpumask
  *
  * Given a node, construct a good cpumask for its sched_domain to span. It
  * should be one that prevents unnecessary balancing, but also spreads tasks