Modify ForeignPtr documentation in light of plusForeignPtr
Modify ForeignPtr documentation in light of plusForeignPtr

Reviewers: austin, rwbarton, simonmar, hvr, bgamari

Reviewed By: rwbarton, simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2970

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