Clarify what hsig writing out is.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Cabal/Distribution/Simple/Build.hs

@@ -595,7 +595,12 @@ writeAutogenFiles verbosity pkg lbi clbi = do
   --TODO: document what we're doing here, and move it to its own function
   case clbi of
     LibComponentLocalBuildInfo { componentInstantiatedWith = insts } ->
-        -- Harmless enough to do things even when they exist
+        -- Write out empty hsig files for all requirements, so that GHC
+        -- has a source file to look at it when it needs to typecheck
+        -- a signature.  It's harmless to write these out even when
+        -- there is a real hsig file written by the user, since
+        -- include path ordering ensures that the real hsig file
+        -- will always be picked up before the autogenerated one.
         for_ (map fst insts) $ \mod_name -> do
             let sigPath = autogenComponentModulesDir lbi clbi
                       </> ModuleName.toFilePath mod_name <.> "hsig"