doc: Document additional RCU task-stall dump information

This commit adds explanation for the added task-stall dumps, which can
be as follows:

	Tasks blocked on level-1 rcu_node (CPUs 0-8): P96/1:b..l.B CPU 5 swapper/5 (0) (idle!)

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>