posix-timers: Document common_clock_get() correctly

Replace another confusing and inaccurate set of comments.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20230425183313.409169321@linutronix.de