Rollup merge of rust-lang#115588 - tifv:fix-comment-successors, r=sco…

…ttmcm

Fix a comment in std::iter::successors

The `unfold` function have since rust-lang#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.)

library/core/src/iter/sources/successors.rs

@@ -17,7 +17,7 @@ where
  F: FnMut(&T) -> Option<T>,
 {
  // If this function returned `impl Iterator<Item=T>`
- // it could be based on `unfold` and not need a dedicated type.
+ // it could be based on `from_fn` and not need a dedicated type.
  // However having a named `Successors<T, F>` type allows it to be `Clone` when `T` and `F` are.
  Successors { next: first, succ }
 }