doc: Add rcutree.rcu_kick_kthreads to kernel-parameters.txt

			Lazy RCU callbacks are those which RCU can

			prove do nothing more than free memory.

	rcutree.rcu_kick_kthreads= [KNL]

			Cause the grace-period kthread to get an extra

			wake_up() if it sleeps three times longer than

			it should at force-quiescent-state time.

			This wake_up() will be accompanied by a

			WARN_ONCE() splat and an ftrace_dump().

	rcuperf.gp_exp= [KNL]

			Measure performance of expedited synchronous

			grace-period primitives.