posix-timers: Clarify posix_timer_fn() comments

Make the issues vs. SIG_IGN understandable and remove the 15 years old
promise that a proper solution is already on the horizon.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/874jnrdmrq.ffs@tglx