Add some documentation

petl/io/db.py

@@ -163,6 +163,7 @@ def _iter_dbapi_cursor(cursor, query, *args, **kwargs):
     cursor.execute(query, *args, **kwargs)
     # fetch one row before iterating, to force population of cursor.description
     # which may be postponed if using server-side cursors
+    # not all database drivers populate cursor after execute so we call fetchall
     try:
         it = iter(cursor)
     except TypeError: