Allow per-argument documentation on pattern synonym signatures
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

ezyang/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc