Since cb5af0d7, some classes that are under Active Storage are now
part of the API site.
However, these classes aren't nested under a definition of the
`ActiveStorage` module but rather name-spaced under it like
`ActiveStorage::Foo`.
Thus, these classes are present both under the ActiveStorage label
and at the root of the site's sidebar so we have to strip out
duplicates.
[ci skip]
Since Active Support is monkey patching a lot of core classes, let's
rather document these changes under a new section so they are still
documented but not encumbering the sidebar.
We can safely remove the rescuing of the `LoadError` since as of
cd7cc525, it's not possible to generate the API from an application.
[ci skip]
[Kasper Timm Hansen & Robin Dupret]
