Rewrite Backpack comments on never-exported TyThings.
Summary: While thesing, I realized this part of the implementation didn't make very much sense, so I started working on some documentation updates to try to make things more explainable. The new docs are organized around the idea of a "never exported TyThing" (a non-implicit TyThing that never occurs in the export list of a module). I also removed some outdated information that predated the change of ModIface to store Names rather than OccNames. Signed-off-by: Edward Z. Yang <email@example.com> Reviewers: simonpj, bgamari, austin Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D2989