Fix associated data family doc structure items

Associated data families were being given their own export DocStructureItems,
which resulted in them being documented separately from their classes in
haddocks. This commit fixes it.

Author: FinleyMcIlwaine

compiler/GHC/Hs/Doc.hs

@@ -123,7 +123,7 @@ type LHsDoc pass = Located (HsDoc pass)
 data DocStructureItem
   = DsiSectionHeading !Int !(HsDoc GhcRn)
   | DsiDocChunk !(HsDoc GhcRn)
-  | DsiNamedChunkRef !(String)
+  | DsiNamedChunkRef !String
   | DsiExports !Avails
   | DsiModExport
       !(NonEmpty ModuleName) -- ^ We might re-export avails from multiple

compiler/GHC/HsToCore/Docs.hs

@@ -192,7 +192,13 @@ mkDocStructureFromDecls env all_exports decls =
         Just loc -> L loc (DsiExports [avail])
         -- FIXME: This is just a workaround that we use when handling e.g.
         -- associated data families like in the html-test Instances.hs.
-        Nothing -> noLoc (DsiExports [avail])
+        Nothing -> noLoc (DsiExports [])
+
+        -- This causes the associated data family to be incorrectly documented
+        -- separately from its class:
+        -- Nothing -> noLoc (DsiExports [avail])
+
+        -- This panics on the associated data family:
         -- Nothing -> panicDoc "mkDocStructureFromDecls: No loc found for"
         --                     (ppr avail)