Allow per-argument documentation on pattern synonym signatures
haddock-2.18 supports user defined pattern synonym signatures so this
seems like a welcomed addition.
Reviewers: austin, bgamari, mpickering
Reviewed By: bgamari, mpickering
Subscribers: rwbarton, thomie, mpickering
Differential Revision: https://phabricator.haskell.org/D3699
(cherry picked from commit 287a40564a7024b55daf0b063683ce889f8a18d9)