tick/nohz: Update obsolete comments

Some comments are obsolete enough to assume that IRQ exit restarts the
tick in idle or RCU is turned on at the same time as the tick, among
other details.

Update them and add more.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230912104406.312185-3-frederic@kernel.org