Clarify AsyncResult.ready docstring (#4038)

CONTRIBUTORS.txt

@@ -236,3 +236,4 @@ Jianjian Yu, 2017/04/09
 Brian May, 2017/04/10
 Dmytro Petruk, 2017/04/12
 Joey Wilhelm, 2017/04/12
+Simon Schmidt, 2017/05/19

celery/result.py

@@ -273,7 +273,7 @@ class AsyncResult(ResultBase):
                     raise IncompleteStream()
 
     def ready(self):
-        """Return :const:`True` if the task started executing.
+        """Return :const:`True` if the task has executed.
 
         If the task is still running, pending, or is waiting
         for retry then :const:`False` is returned.