bpo-32208: update threading.Semaphore docs and add unit test (GH-4709) (#4750)

bpo-32208: update threading.Semaphore docs and add unit test (GH-4709) (#4750)

* 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
(cherry picked from commit a0374dd34aa25f0895195d388b5ceff43b121b00)