bpo-32208: update threading.Semaphore docs and add unit test (#4709)

bpo-32208: update threading.Semaphore docs and add unit test (#4709)

* fix issue32208: update threading.Semaphore docs and add unit test to validate correct behavior

* add test for blocking

* Update threading.rst

* semaphore: remove documentation validation tests and move 'return value' test to BaseSemaphore