rcu: Update RCU's trace.txt documentation for new format

The trace.txt file had obsolete output for the debugfs rcu/rcudata
file, so update it.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>