Change ray.worker.cleanup -> ray.shutdown and improve API documentation. (#2374)

* Change ray.worker.cleanup -> ray.shutdown and improve API documentation.

* Deprecate ray.worker.cleanup() gracefully.

* Fix linting

python/ray/rllib/tuned_examples/regression_tests/regression_test.py

@@ -43,7 +43,7 @@ class Regression():
         raise NotImplementedError
 
     def teardown(self, *args):
-        ray.worker.cleanup()
+        ray.shutdown()
 
     def track_time(self, result):
         return result["time_total_s"]