tick/sched: Update tick_sched struct documentation

Adapt the documentation order of struct members to the effective order of
struct members and add missing descriptions.

Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: fweisbec@gmail.com
Cc: peterz@infradead.org
Link: https://lkml.kernel.org/r/20190321120921.16463-2-anna-maria@linutronix.de