posix-timers: Document sys_clock_getres() correctly

The decades old comment about Posix clock resolution is confusing at best.

Remove it and add a proper explanation to sys_clock_getres().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20230425183313.356427330@linutronix.de