patch applied (ghc-fc): Make sym coercion smart constructor smarter, add comments

[Kevin Donnelly]

Tue Aug  1 08:30:14 PDT 2006  kevind at bu.edu
  * Make sym coercion smart constructor smarter, add comments

    M ./compiler/typecheck/TcGadt.lhs -2 +4
    M ./compiler/types/Coercion.lhs -5 +27