Rollup merge of #115588 - tifv:fix-comment-successors, r=scottmcm

Fix a comment in std::iter::successors

The `unfold` function have since #58062 been renamed to `from_fn`.
(I'm not sure if this whole comment is still useful—it's not like there are many iterators that *can't* be based on `from_fn`. Anyway, in its current form this comment is not correct, and it sent me into a half-hour research of what happened to `unfold` function, so I want to do *something* with it 🙃 deleting these three lines is a perfectly fine alternative, in my opinion.)

Author: GuillaumeGomez