genirq: irq_desc: Document preflow_handler and affinity_hint

[ tglx: Filled in the FIXME place holders ]

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: http://lkml.kernel.org/r/%3C1302426113-13808-2-git-send-email-geert%40linux-m68k.org%3E


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