This is a continuation of
   commit e9e61f18
   Date:   Thu May 26 15:24:53 2016 +0100
   Reduce special-casing for nullary unboxed tuple

which related to Trac #12115.  But typecheck/should_run/tcrun051
revealed that my patch was incomplete.

This fixes it, by removing another special case in Type.repType.
I had also missed a case in UnariseStg.unariseIdBinder.

I took the opportunity to add explanatory notes
  Note [Unarisation]
  Note [Unarisation and nullary tuples]
in UnariseStg