Distinction between static and dynamic handlers unclear in documentation

[corranwebster]

In https://github.com/enthought/traits/blob/master/docs/source/traits_user_manual/notification.rst the description of the @on_trait_change decorator is misleading in that it claims to generate static trait change handlers, but the reality is more complex.

[kitchoi]

The documentation referenced in this issue has since been moved: https://docs.enthought.com/traits/traits_user_manual/listening.html