Fix documentation of new controller filters api [ci skip]

The api for filters with classes change and the guides weren't updated.
Now the class must respond for methods with the same name as the filter,
so the `before_action` calls a `before` method, and so on.

The method `#filter` has been deprecated in 4.0.0 and has been removed
in 4.1.0: #7560
Merge pull request #13871 from kassio/fix-filters-with-classes-documentation

Fix documentation of filters api with classes [ci skip]