doc: clearer and more correct Iterator::scan

The `Iterator::scan` documentation seemed a little misleading to my newcomer
eyes, and this tries to address that.

* I found “similar to `fold`” unhelpful because (a) the similarity is only that
  they maintain state between iterations, and (b) the _dissimilarity_ is no less
  important: one returns a final value and the other an iterator. So this
  replaces that with “which, like `fold`, holds internal state, but unlike
  `fold`, produces a new iterator.

* I found “the return value from the closure, an [`Option`], is yielded by the
  iterator” to be downright incorrect, because “yielded by the iterator” means
  “returned by the `next` method wrapped in `Some`”, so this implied that `scan`
  would convert an input iterator of `T` to an output iterator of `Option<T>`.
  So this replaces “yielded by the iterator” with “returned by the `next`
  method” and elaborates: “Thus the closure can return `Some(value)` to yield
  `value`, or `None` to end the iteration.”

* This also changes the example to illustrate the latter point by returning
  `None` to terminate the iteration early based on `state`.

Author: gthb

library/core/src/iter/traits/iterator.rs

@@ -1380,8 +1380,8 @@ pub trait Iterator {
         Take::new(self, n)
     }
 
-    /// An iterator adapter similar to [`fold`] that holds internal state and
-    /// produces a new iterator.
+    /// An iterator adapter which, like [`fold`], holds internal state, but
+    /// unlike [`fold`], produces a new iterator.
     ///
     /// [`fold`]: Iterator::fold
     ///
@@ -1393,20 +1393,25 @@ pub trait Iterator {
     ///
     /// On iteration, the closure will be applied to each element of the
     /// iterator and the return value from the closure, an [`Option`], is
-    /// yielded by the iterator.
+    /// returned by the `next` method. Thus the closure can return
+    /// `Some(value)` to yield `value`, or `None` to end the iteration.
     ///
     /// # Examples
     ///
     /// Basic usage:
     ///
     /// ```
-    /// let a = [1, 2, 3];
+    /// let a = [1, 2, 3, 4];
     ///
     /// let mut iter = a.iter().scan(1, |state, &x| {
-    ///     // each iteration, we'll multiply the state by the element
+    ///     // each iteration, we'll multiply the state by the element ...
     ///     *state = *state * x;
     ///
-    ///     // then, we'll yield the negation of the state
+    ///     // ... and terminate if the state exceeds 6
+    ///     if (*state > 6) {
+    ///         return None;
+    ///     }
+    ///     // ... else yield the negation of the state
     ///     Some(-*state)
     /// });
     ///