message: Add a quick note on row polymorphism
diff: @@ -320,6 +320,9 @@ Two typesets `A` and `B` are defined to be structurally equal if `A <: B` and
  > `<:` so we need to work out if we can elide them from the surface language.
  > This requires considering polymorphic function arguments, partial data,
  > qualified function types and variable definitions.
  +> - Reformulate this in terms of row polymorphism. We really want to avoid a
  +> _real_ subtyping relationship as it doesn't play at all well with global
  +> inference.
  #### Unsafe Typeset Field Mutation
  For performance it is sometimes necessary to have the ability to _directly_
  