rcu: Fix srcu_barrier() docbook header
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 <1e0ce936bb9b355d257bf5790d2513c3f28be22b@linux.vnet.ibm.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs