Make :doc ModuleName print module docs.

E.g.

ghci> :doc Prelude
-- |
-- Module      :  Prelude
-- Copyright   :  (c) The University of Glasgow 2001
-- License     :  BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer  :  libraries@haskell.org
-- Stability   :  stable
-- Portability :  portable
--
-- The Prelude: a standard module. The Prelude is imported by default
-- into all Haskell modules unless either there is an explicit import
-- statement for it, or the NoImplicitPrelude extension is enabled.
--

as :doc prints docstrings as is (which is fine).