Fix confusing doc for `scan`

Fix confusing doc for `scan`

The comment "the value passed on to the next iteration" confused me since it sounded more like what Haskell's [scanl](http://hackage.haskell.org/package/base-4.11.0.0/docs/Prelude.html#v:scanl) does where the closure's return value serves as both the "yielded value" *and* the new value of the "state".

I tried changing the example to make it clear that the closure's return value is decoupled from the state argument.