Document Coercible in the user guide
Document Coercible in the user guide

as a subsection of "Equality constraints", containing references to the
module's haddock and to the paper. Fixes #8888

(cherry picked from commit 1e36a386042248523de69ad6b02c43a6631ed5d0)

lukexi/ghc-7.8-arm64,hferreiro/replay,lukexi/ghc-7.8-arm64,lukexi/ghc-7.8-arm64,lukexi/ghc-7.8-arm64,hferreiro/replay,lukexi/ghc-7.8-arm64,hferreiro/replay,hferreiro/replay,hferreiro/replay,hferreiro/replay,hferreiro/replay