Add new footnote to clarify the meaning of "affected rows" in the DB-API.

See DB-SIG mailing list for a discussion:

http://mail.python.org/pipermail/db-sig/2012-June/005856.html

pep-0249.txt

@@ -287,7 +287,7 @@ Cursor Objects
             This read-only attribute specifies the number of rows that
             the last .execute*() produced (for DQL statements like
             'select') or affected (for DML statements like 'update' or
-            'insert').
+            'insert'). [9]
             
             The attribute is -1 in case no .execute*() has been
             performed on the cursor or the rowcount of the last
@@ -1127,6 +1127,20 @@ Footnotes
         implement the tp_iter slot on the cursor object instead of the
         .__iter__() method.
 
+    [9] The term "number of affected rows" generally refers to the
+        number of rows deleted, updated or inserted by the last
+        statement run on the database cursor. Most databases will
+        return the total number of rows that were found by the
+        corresponding WHERE clause of the statement. Some databases
+        use a different interpretation for UPDATEs and only return the
+        number of rows that were changed by the UPDATE, even though
+        the WHERE clause of the statement may have found more matching
+        rows. Database module authors should try to implement the more
+        common interpretation of returning the total number of rows
+        found by the WHERE clause, or clearly document a different
+        interpretation of the rowcount attribute.
+
+
 Acknowledgements
 
     Many thanks go to Andrew Kuchling who converted the Python