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)