SF bug #964230: random.choice([]) should return more intelligible exception

Add a comment to make the traceback less mysterious.

Lib/random.py

@@ -237,7 +237,7 @@ class Random(_random.Random):
 
     def choice(self, seq):
         """Choose a random element from a non-empty sequence."""
-        return seq[int(self.random() * len(seq))]
+        return seq[int(self.random() * len(seq))]  # raises IndexError in seq is empty
 
     def shuffle(self, x, random=None, int=int):
         """x, random=random.random -> shuffle list x in place; return None.