Added clarification about use of .executemany() with stored procedures.

Patch by Bob Kline.

pep-0249.txt

@@ -337,6 +337,12 @@ Cursor Objects
             to have the database process the sequence as a whole in
             one call.
             
+            Use of this method for an operation which produces one or
+            more result sets constitutes undefined behavior, and the
+            implementation is permitted (but not required) to raise 
+            an exception when it detects that a result set has been
+            created by an invocation of the operation.
+            
             The same comments as for execute() also apply accordingly
             to this method.