doc: Add rcutree.kthread_prio pointer to stallwarn.txt

This commit adds mention of the rcutree.kthread_prio kernel boot parameter
to the discussion of how high-priority real-time tasks can result in
RCU CPU stall warnings.  (However, this does not necessarily help when
the high-priority real-time tasks are using dubious deadlines.)

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>