nohz: Clarify magic in tick_nohz_stop_sched_tick()

While going through the nohz code I got stumped by some of it.

This patch adds a few comments clarifying the code; based on discussion
with Thomas.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/20151119162106.GO3816@twins.programming.kicks-ass.net


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>