std: fix doctest and explain for as_slices and as_mut_slices in VecDeque

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

Author: xizheyin

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

@@ -1312,6 +1312,8 @@ 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.
+/// Otherwise, the exact split point depends on implementation details
+/// and is not guaranteed.
 ///
 /// [`make_contiguous`]: VecDeque::make_contiguous
 ///
@@ -1326,12 +1328,18 @@ impl<T, A: Allocator> VecDeque<T, A> {
 /// deque.push_back(1);
 /// deque.push_back(2);
 ///
-/// assert_eq!(deque.as_slices(), (&[0, 1, 2][..], &[][..]));
+/// let expected = [0, 1, 2];
+/// let (front, back) = deque.as_slices();
+/// assert_eq!(&expected[..front.len()], front);
+/// assert_eq!(&expected[front.len()..], back);
 ///
 /// deque.push_front(10);
 /// deque.push_front(9);
 ///
-/// assert_eq!(deque.as_slices(), (&[9, 10][..], &[0, 1, 2][..]));
+/// let expected = [9, 10, 0, 1, 2];
+/// let (front, back) = deque.as_slices();
+/// assert_eq!(&expected[..front.len()], front);
+/// assert_eq!(&expected[front.len()..], back);
 /// ```
 #[inline]
 #[stable(feature = "deque_extras_15", since = "1.5.0")]
@@ -1347,6 +1355,8 @@ 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.
+/// Otherwise, the exact split point depends on implementation details
+/// and is not guaranteed.
 ///
 /// [`make_contiguous`]: VecDeque::make_contiguous
 ///
@@ -1363,9 +1373,22 @@ impl<T, A: Allocator> VecDeque<T, A> {
 /// deque.push_front(10);
 /// deque.push_front(9);
 ///
-/// deque.as_mut_slices().0[0] = 42;
-/// deque.as_mut_slices().1[0] = 24;
-/// assert_eq!(deque.as_slices(), (&[42, 10][..], &[24, 1][..]));
+/// // Since the split point is not guaranteed, we may need to update
+/// // either slice.
+/// let mut update_nth = |index: usize, val: u32| {
+/// let (front, back) = deque.as_mut_slices();
+/// if index > front.len() - 1 {
+/// back[index - front.len()] = val;
+/// } else {
+/// front[index] = val;
+/// }
+/// };
+///
+/// update_nth(0, 42);
+/// update_nth(2, 24);
+///
+/// let v: Vec<_> = deque.into();
+/// assert_eq!(v, [42, 10, 24, 1]);
 /// ```
 #[inline]
 #[stable(feature = "deque_extras_15", since = "1.5.0")]