Improve doc example for `std::borrow::Cow`.

src/libcollections/borrow.rs

@@ -86,16 +86,29 @@ impl<T> ToOwned for T where T: Clone {
 /// ```
 /// use std::borrow::Cow;
 ///
-/// # #[allow(dead_code)]
 /// fn abs_all(input: &mut Cow<[i32]>) {
 ///     for i in 0..input.len() {
 ///         let v = input[i];
 ///         if v < 0 {
-///             // clones into a vector the first time (if not already owned)
+///             // Clones into a vector if not already owned.
 ///             input.to_mut()[i] = -v;
 ///         }
 ///     }
 /// }
+///
+/// // No clone occurs because `input` doesn't need to be mutated.
+/// let slice = [0, 1, 2];
+/// let mut input = Cow::from(&slice[..]);
+/// abs_all(&mut input);
+///
+/// // Clone occurs because `input` needs to be mutated.
+/// let slice = [-1, 0, 1];
+/// let mut input = Cow::from(&slice[..]);
+/// abs_all(&mut input);
+///
+/// // No clone occurs because `input` is already owned.
+/// let mut input = Cow::from(vec![-1, 0, 1]);
+/// abs_all(&mut input);
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub enum Cow<'a, B: ?Sized + 'a>