Replace fragile GHC manual section references by urls

This replaces the GHC/Hugs manual references in the Haddock comments
for the `Language.Haskell.Extension.Extension` enum constructors, as
in

```haskell
  -- | [GHC § 7.6.3.4] Allow overlapping class instances,
  -- provided there is a unique most specific instance for each use.
  OverlappingInstances
```

which urls pointing directly to the section containing further
documentation (assuming the urls are more immune to section
reorderings), e.g.:

```haskell
  -- | 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>
     OverlappingInstances
```