Bugfix in TWeakRef._check_clean
Add changes_only_according_to, a variant of changes_only that allows the comparison predicate to be specified, and make changes_only always hold strong refs for now

The logic behind only holding strong refs is that holding weak refs could really screw with a potential use of changes_only_according_to(operator.eq): allowing the watched function to be one that produces anonymous tuples of values, which for obvious reasons would have no references themselves after being passed through the callback.
Get rid of redundant documentation