rcu: Fix srcu_barrier() docbook header

The srcu_barrier() docbook header left out the "sp" argument, so this
commit adds that argument's docbook text.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/srcu.c

@@ -491,6 +491,7 @@ EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
 
 /**
  * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
+ * @sp: srcu_struct on which to wait for in-flight callbacks.
  */
 void srcu_barrier(struct srcu_struct *sp)
 {