document invariant of VecDeque::as_(mut)_slice

Author: lcnr

src/liballoc/collections/vec_deque.rs

@@ -959,6 +959,9 @@ impl<T> VecDeque<T> {
     /// Returns a pair of slices which contain, in order, the contents of the
     /// `VecDeque`.
     ///
+    /// In case [`make_contiguous`](#method.make_contiguous) was previously called,
+    /// all elements of the deque are in the first slice and the second slice is empty.
+    ///
     /// # Examples
     ///
     /// ```
@@ -989,6 +992,9 @@ impl<T> VecDeque<T> {
     /// Returns a pair of slices which contain, in order, the contents of the
     /// `VecDeque`.
     ///
+    /// In case [`make_contiguous`](#method.make_contiguous) was previously called,
+    /// all elements of the deque are in the first slice and the second slice is empty.
+    ///
     /// # Examples
     ///
     /// ```