Document that bundled pattern synonyms have only been supported since 8.0.1

Bundled pattern synonyms like in the example below have only been supported since 8.0.1.

{-# language PatternSynonyms #-}
module Lib (D(P)) where

data D = C

pattern P = C

The current documentation is in a section marked :since: 7.8.1, which seems to imply that bundled exports have also been available since 7.8.1.