rcu: update tracing documentation for new rcutorture and rcuboost

This commit documents the new debugfs rcu/rcutorture and rcu/rcuboost
trace files.  The description has been updated as suggested by Josh
Triplett.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>