[docs] Incorrect information for multiprocessing.Semaphore

There's a Note on the documentation for multiprocessing.Semaphore that says that sem_timedwait is unsupported on OS/X and that any timeout argument provided to acquire() will be ignored. I checked the Modules/_multiprocessing/semaphore.c code on the 2.6, 3.0, and 3.1 branches and I see that the module will fallback to sem_timedwait_save() if sem_timedwait is not available. That is, it will not ignore the timeout, but will instead emulate a timed wait using a sleep loop.