This replaces the GHC/Hugs manual references in the Haddock comments
for the `Language.Haskell.Extension.Extension` enum constructors, as
-- | [GHC § 22.214.171.124] Allow overlapping class instances,
-- provided there is a unique most specific instance for each use.
which urls pointing directly to the section containing further
documentation (assuming the urls are more immune to section
-- | Allow overlapping class instances, provided there is a unique
-- most specific instance for each use.
-- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#instance-overlap>