Documentation of CoAxiom: saturation and roles

The documentation around CoAxiom has a couple of problems: It is inconsistently claimed that

• CoAxioms are applied to Nominal coercions only (https://gitlab.haskell.org/ghc/ghc/blob/master/compiler/types/TyCoRep.hs#L1196),
• and also that they have role annotations similar to TyCons (https://gitlab.haskell.org/ghc/ghc/blob/master/compiler/types/CoAxiom.hs#L333).

I think the second of these is the correct one.

Secondly, the note [CoAxiom saturation] (https://gitlab.haskell.org/ghc/ghc/blob/master/compiler/types/CoAxiom.hs#L313) is rather truncated, and I couldn't see anything in the git history that should have gone there

Environment

• GHC version used: 8.6.5, master