documentation: Update rcutree.kthread_prio for grace-period kthread use

Now that the rcutree.kthread_prio kernel boot parameter also controls
the priority of the grace-period kthreads, update the documentation to
reflect this change.

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

diff --cc Documentation/kernel-parameters.txt
Simple merge