rcu: refer RCU CPU stall-warning victims to stallwarn.txt

There is some documentation on RCU CPU stall warnings contained in
Documentation/RCU/stallwarn.txt, but it will not be apparent to someone
who runs into such a warning while under time pressure.  This commit
therefore adds comments preceding the printk()s pointing out the
location of this documentation.

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