Further fixes in RnNames, to make associated type exports work

You ought to be able to say

  module M( C( T, foo ) where
    class C a where
      type T a
      foo :: a -> T a

i.e. with T in C's sub-item list.  This makes it so.
Document -XTypeOperators, which had escaped documentation altogether thus far
Make noteMustPointToIt true of all non-top-level thunks

See Note [GC recovery].  To come: clean-up of StgCmmBind.cgRhs.