Extend the comment about the hack

Author: Mikolaj

Cabal/src/Distribution/Simple/Hpc.hs

@@ -71,8 +71,13 @@ mixDir :: FilePath  -- ^ \"dist/\" prefix
 mixDir distPref way name = hpcDir distPrefBuild way </> "mix" </> name
  where
   -- This is a hack for HPC over test suites, needed to match the directory
-  -- where HPC stores .mix files when the main library of the same package
-  -- is being processed, perhaps in a previous cabal run (#5213).
+  -- where HPC saves and reads .mix files when the main library of the same
+  -- package is being processed, perhaps in a previous cabal run (#5213).
+  -- E.g., @distPref@ may be
+  -- @./dist-newstyle/build/x86_64-linux/ghc-9.0.1/cabal-gh5213-0.1/t/tests@
+  -- but the path where library mix files reside has two less components
+  -- at the end (@t/tests@) and this reduced path needs to be passed to
+  -- both @hpc@ and @ghc@.
   distPrefElements = splitDirectories distPref
   distPrefBuild = case drop (length distPrefElements - 2) distPrefElements of
     "t" : _ -> joinPath $ take (length distPrefElements - 2) distPrefElements