`cabal new-haddock` doesn't generate docs for Backpack mixins

I don't know if this is a cabal issue or a haddock issue.

Modules that I generate via the `mixin` field in my .cabal don't have any docs generated for them. This is a big deal because I have a project in which many modules depend on a couple of signatures, so none of these modules have documentation generated.

For an example, see https://github.com/LightAndLight/haddock-backpack-issue