You can't.
What you can do is document your instance.
-- | You can have a brief description here
instance (Monad m) => MonadState s (ChronoT s e m) where
…
This will make the description show up to the side of the instances
box generated. This should preferably be brief but it does let you do things such as point the user to documentation of the functions implementing the fields if you really need to, like the commenter on the question suggests.
Personally, I think that it doesn't make much sense to document each field like this: what the fields are meant to do should be clear from documentation of the class definition and the comment on the instance.