Unrendered `@since` annotations

Originally opened at https://github.com/haskell/haddock/issues/1577 by @treeowl:

I just noticed that the rendered documentation for Data.Semigroup is missing @since annotations for Generic and Generic1 instances. I figured that someone must have forgotten to put them in, but that is not the case! See, for example, the documentation for Last vs. the source for Last. While all the derived instances have @since annotations in the source, not all of them are rendered.