Docs: datacon eta-expansion, rep-poly checks

The existing notes weren't very clear on how the eta-expansion of
data constructors that occurs in tcInferDataCon/dsConLike interacts
with the representation polymorphism invariants. So we explain with
a few more details how we ensure that the representation-polymorphic
lambdas introduced by tcInferDataCon/dsConLike don't end up causing
problems, by checking they are properly instantiated and then relying
on the simple optimiser to perform beta reduction.

A few additional changes:

  - ConLikeTc just take type variables instead of binders, as we
    never actually used the binders.
  - Removed the FRRApp constructor of FRROrigin; it was no longer used
    now that we use ExpectedFunTyOrigin.
  - Adds a bit of documentation to the constructors
    of ExpectedFunTyOrigin.