doc: Now jiffies_till_sched_qs solicits help from cond_resched()

The rcutree.jiffies_till_sched_qs kernel boot parameter used to solicit
help only from rcu_note_context_switch(), but now also solicits help
from cond_resched().  This commit therefore updates kernel-parameters.txt
accordingly.

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