What does the star (*) in instance documentation mean?

Question

In the MonadState doc page, there's a star (*) inside the ReaderT type of the MonadState instance:

MonadState s m => MonadState s (ReaderT * r m)

But actually in the source, there's no star:

instance MonadState s m => MonadState s (ReaderT r m) where

So what does the star mean in the doc?