rcu: Update jiffies_to_sched_qs and adjust_jiffies_till_sched_qs() comments

This commit better documents the jiffies_to_sched_qs default-value
strategy used by adjust_jiffies_till_sched_qs()

Reported-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>