Improve Iterator::by_ref example

I split the example into two: one that fails to compile, and one that
works. I also made them identical except for the addition of `by_ref`
so we don't confuse readers with random differences.

Author: camelid

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

@@ -1535,34 +1535,32 @@ pub trait Iterator {
     ///
     /// # Examples
     ///
-    /// Basic usage:
+    /// This demonstrates a use case that needs `by_ref`:
     ///
-    /// ```
-    /// let a = [1, 2, 3];
-    ///
-    /// let iter = a.iter();
-    ///
-    /// let sum: i32 = iter.take(5).fold(0, |acc, i| acc + i);
+    /// ```compile_fail,E0382
+    /// let a = [1, 2, 3, 4, 5];
+    /// let mut iter = a.iter();
+    /// let sum: i32 = iter.take(3).fold(0, |acc, i| acc + i);
     ///
     /// assert_eq!(sum, 6);
     ///
-    /// // if we try to use iter again, it won't work. The following line
-    /// // gives "error: use of moved value: `iter`
-    /// // assert_eq!(iter.next(), None);
+    /// // Error! We can't use `iter` again because it was moved
+    /// // by `take`.
+    /// assert_eq!(iter.next(), Some(&4));
+    /// ```
     ///
-    /// // let's try that again
-    /// let a = [1, 2, 3];
+    /// Now, let's use `by_ref` to make this work:
     ///
+    /// ```
+    /// let a = [1, 2, 3, 4, 5];
     /// let mut iter = a.iter();
+    /// // We add in a call to `by_ref` here so `iter` isn't moved.
+    /// let sum: i32 = iter.by_ref().take(3).fold(0, |acc, i| acc + i);
     ///
-    /// // instead, we add in a .by_ref()
-    /// let sum: i32 = iter.by_ref().take(2).fold(0, |acc, i| acc + i);
-    ///
-    /// assert_eq!(sum, 3);
+    /// assert_eq!(sum, 6);
     ///
-    /// // now this is just fine:
-    /// assert_eq!(iter.next(), Some(&3));
-    /// assert_eq!(iter.next(), None);
+    /// // And now we can use `iter` again because we still own it.
+    /// assert_eq!(iter.next(), Some(&4));
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     fn by_ref(&mut self) -> &mut Self