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some outdated information that predated the change of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 <ezyang@cs.stanford.edu>

Reviewers: simonpj, bgamari, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2989