Update docs

Author: fpringle

src/Generics/Diff/Class.hs

@@ -34,6 +34,10 @@ we can "descend" through) depends on the implementation of the instance.
 For user-defined types, it's strongly recommended you derive your 'Diff' instance using 'Generic' from
 @generics-sop@. If those types refer to other types, those will need 'Diff' instances too. For example:
 
+However, in some cases we'll want to use a custom type for representing diffs of user-defined or
+third-party types. For example, if we have non-derived `Eq` instances, invariants etc. In that case,
+see "Generics.Diff.Special".
+
 @
 {\-# LANGUAGE DerivingStrategies #-\}
 {\-# LANGUAGE DeriveGeneric #-\}

src/Generics/Diff/Type.hs

@@ -19,7 +19,8 @@ newtype Differ x = Differ (x -> x -> DiffResult x)
 
 {- | A GADT representing an error during the diff algorithm - i.e. this tells us where and how two values differ.
 
-The special constructors for list are so that we can treat these types a bit uniquely. See 'ListDiffError'.
+The 'DiffSpecial' constructors for instances of 'SpecialDiff' are so that we can treat these types uniquely.
+See 'SpecialDiff'.
 -}
 data DiffError a where
   -- | All we can say is that the values being compared are not equal.