jiffies: Transform comment about time_* functions into DOC block

This general note about time_* functions is also useful to be available in
kernel documentation. Therefore transform it into a kernel-doc DOC block
with proper formatting.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240123164702.55612-6-anna-maria@linutronix.de