document: fix trac issue #14229
document: fix trac issue #14229

Accroding to
https://git.haskell.org/ghc.git/commitdiff/49672659113371c3bee691e6d913d
f8e6f60a1d8,
`-Wredundant-constraints` is no longer turn on by default.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4528

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc