Haddock comments to class arguments

[Lev135]

I would like to add a haddock comment to class arguments like this:

class C 
  (a :: Type) -- ^ comment to a
  (b :: Constraint) -- ^ comment to b
  where

or (with -XStandaloneKindSignatures)

type C
  :: Type -- ^ comment to a
  -> Constraint -- ^ comment to b
  -> Constraint
class C a b where

However, as far as I can see, it's not possible in current version