std: chain the slices for VecDeque::as_slice doctest and add explanation

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

Author: xizheyin

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

@@ -1313,6 +1313,9 @@ 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
@@ -1326,12 +1329,14 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// deque.push_back(1);
     /// deque.push_back(2);
     ///
-    /// assert_eq!(deque.as_slices(), (&[0, 1, 2][..], &[][..]));
+    /// let slices = deque.as_slices();
+    /// assert!(slices.0.iter().chain(slices.1.iter()).eq([0, 1, 2].iter()));
     ///
     /// deque.push_front(10);
     /// deque.push_front(9);
     ///
-    /// assert_eq!(deque.as_slices(), (&[9, 10][..], &[0, 1, 2][..]));
+    /// let slices = deque.as_slices();
+    /// assert!(slices.0.iter().chain(slices.1.iter()).eq([9, 10, 0, 1, 2].iter()));
     /// ```
     #[inline]
     #[stable(feature = "deque_extras_15", since = "1.5.0")]