Fix docs: AR after/around callback returning false

Currently, the [ActiveRecord::Callbacks documentation](http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html#module-ActiveRecord::Callbacks-label-Canceling+callbacks) states:

> If a `before_*` callback returns `false`, all the later callbacks and the associated action are
> cancelled. If an `after_*` callback returns `false`, all the later callbacks are cancelled.

However, the last statement is not true: returning `false` in an `after_` callback in ActiveRecord does not prevent further `after_` callbacks from being executed.

This commit fixes the documentation and adds a test that shows how returning `false` both in an `after_save` and in an `around_save` does not prevent further callbacks from being executed.