Fix w.r.t. exception tuples.

pep-3110.txt

@@ -83,7 +83,7 @@ to ::
 
 The use of ``as`` in place of the comma token means that ::
 
-    except AttributeError, os.error:
+    except (AttributeError, os.error):
     
 can be clearly understood as a tuple of exception classes. This new
 syntax was first proposed by Greg Ewing [#firstproposal]_ and
@@ -93,6 +93,14 @@ Further, the restriction of the token following ``as`` from ``test``
 to ``NAME`` means that only valid identifiers can be used as
 ``except`` targets.
 
+Note that the grammar above always requires parenthesized tuples as
+exception clases. That way, the ambiguous ::
+
+    except A, B:
+
+which would mean different things in Python 2.x and 3.x -- leading to
+hard-to-catch bugs -- cannot legally occur in 3.x code.
+
 
 Semantic Changes
 ================