Document history attribute of table handler

trytond/doc/ref/backend.rst

@@ -253,6 +253,10 @@ TableHandler
    Handle table for the ``model``.
    If ``history`` is set, the table is the one storing the history.
 
+.. attribute:: TableHandler.history
+
+   A boolean to handle the history table.
+
 .. attribute:: TableHandler.namedatalen
 
    The maximing length of named data for the database.