Remove specific instructions on making custom types an instance of Hashable and

instead just point out to documentation.

Author: m-renaud

Data/HashSet.hs

@@ -60,18 +60,15 @@ fromList ["Map","HashSet","Graph","HashMap","Set","Sequence"]
 
 === Using custom data with HashSet
 
-To create a @HashSet@ of your custom type, the type must have instances for 'Eq' and
-'Hashable'. The 'Hashable' typeclass is defined in the
-<https://hackage.haskell.org/package/hashable hashable> package and the
-recommended way to define the instance is using generics, for which you'll need
-the @DeriveGeneric@ GHC extension.
-
-Note: You'll need to enable the @DeviceGeneric@ language extension, either via
-@:set -XDeriveGeneric@ in GHCi or @\{\-\# DeriveGeneric \#\-\}@ in your Haskell source
-code.
+To create a @HashSet@ of your custom type, the type must have instances for 'Eq'
+and 'Hashable'. The 'Hashable' typeclass is defined in the
+<https://hackage.haskell.org/package/hashable hashable> package, see the
+documentation for information on how to make your type an instance of
+'Hashable'.
 
 We'll start by setting up our custom data type:
 
+>>> :set -XDeriveGeneric
 >>> import GHC.Generics (Generic)
 >>> import Data.Hashable
 >>> data Person = Person { name :: String, likesDogs :: Bool } deriving (Show, Eq, Generic)