Named documentation chunks in module headers

Originally opened at https://github.com/haskell/haddock/issues/965 by @sgraf812

Apparently, haddock (or the parser, rather) doesn't allow named chunk references in the module header:

-- |
-- $doc
module Foo where

-- $doc
-- foo

Foo.hs:2:1: error: parse error on input ‘-- $doc’
  |
2 | -- $doc
  | ^^^^^^^