Remove awkward use of undefined from Generics usage example in docs

Summary

Location of documentation issue: GHC Generics docs

The docs for the GHC.Generics module include a nice little example of binary serialization for arbitrary Generic datatypes. As part of this example, an instance for V1 is introduced as follows:

-- The instance for 'V1' is slightly awkward (but also rarely used):
--
-- @
-- instance Encode' 'V1' where
--   encode' x = undefined
-- @
--
-- There are no values of type @V1 p@ to pass (except undefined), so this is actually impossible. ...

Somewhat confusingly, the undefined that occurs in the snippet is not the same as the one referred to in the comment. I think this would be a little less awkward if the documentation instead read...

Proposed improvements or changes

-- The instance for 'V1' is as follows (and requires the pragma @EmptyCase@):
--
-- @
-- instance Encode' 'V1' where
--   encode' x = case x of { }
-- @
--
-- There are no values of type @V1 p@ to pass, so it is impossible for this
-- function to be invoked. Thus we need no cases to handle the input. ...