Currently, documentation strings on function arguments has to be written
after the argument (i.e., using `{-^ -}` comments).  This patch allows
us to use `{-| -}` comments to put the comment string before an
argument.   The same works for the results of functions.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, mpickering, carter

Differential Revision: https://phabricator.haskell.org/D4767