clear up `isdone` docstring (#58958)
I got pretty confused on my first reading of this docstring because for
some reason I thought it was saying that `isdone(itr, state) == missing`
implied that it was true that `iterate(itr, state) === nothing` (aka
that `state` is indeed final). which of course is wrong and doesn't make
sense, but it's still how I read it. I think the new docstring is a bit
more explicit.
