rcu: Update documentation for TREE_RCU debugfs tracing

This commit updates the tracing documentation to reflect the new
format that has per-RCU-flavor directories.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>