[selectors] match_kwargs and match_form_params selectors both accept a new `once_is_enough` argument (closes #1990438) also fix docstring of match_exception and references to selectors in the book.

if schema['TZDatetime'].eid is None:
    add_entity_type('TZDatetime')
if schema['TZTime'].eid is None:
    add_entity_type('TZTime')