Haddock comments on constructor arguments

[waern]

We could try to support Haddock comments on individual constructor arguments:

data A = A Int   -- ^ Documentation for the Int argument
           Float -- ^ Documentation for the Float argument
       | B

But I'm not sure how, since we already support commenting constructors on the same line:

data T a b
  = C1 a b  -- ^ This is the documentation for the 'C1' constructor
  | C2 a b  -- ^ This is the documentation for the 'C2' constructor

Perhaps we could require a parenthesis around the last argument?

data A = A Int   -- ^ Documentation for the Int argument
           (Float -- ^ Documentation for the Float argument)
       | B

Or around both arguments, for consistency:

data A = A (Int   -- ^ Documentation for the Int argument)
           (Float -- ^ Documentation for the Float argument)
       | B