std: test head or tail for VecDeque::as_slice_mut doctest and add explanation

Signed-off-by: xizheyin <[email protected]>

Author: xizheyin

library/alloc/src/collections/vec_deque/mod.rs

@@ -1353,6 +1353,9 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// If [`make_contiguous`] was previously called, all elements of the
     /// deque will be in the first slice and the second slice will be empty.
     ///
+    /// Note that the particular split point between the slices is not guaranteed
+    /// and may change based on internal implementation details.
+    ///
     /// [`make_contiguous`]: VecDeque::make_contiguous
     ///
     /// # Examples
@@ -1362,15 +1365,20 @@ impl<T, A: Allocator> VecDeque<T, A> {
     ///
     /// let mut deque = VecDeque::new();
     ///
-    /// deque.push_back(0);
     /// deque.push_back(1);
+    /// deque.push_back(2);
+    /// deque.push_back(3);
+    /// deque.push_back(4);
     ///
-    /// deque.push_front(10);
-    /// deque.push_front(9);
+    /// let (front, back) = deque.as_mut_slices();
     ///
-    /// deque.as_mut_slices().0[0] = 42;
-    /// deque.as_mut_slices().1[0] = 24;
-    /// assert_eq!(deque.as_slices(), (&[42, 10][..], &[24, 1][..]));
+    /// if !front.is_empty() {
+    ///     front[0] = 5;
+    ///     assert_eq!(deque[0], 5);
+    /// } else {
+    ///     back[back.len() - 1] = 5;
+    ///     assert_eq!(deque[deque.len() - 1], 5);
+    /// }
     /// ```
     #[inline]
     #[stable(feature = "deque_extras_15", since = "1.5.0")]