add example to prepend

Author: antonilol

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

@@ -1975,7 +1975,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// The order of the contents is preserved.
     ///
     /// To get behavior like [`append`][VecDeque::append] where elements are moved
-    /// from the other collection to this one, use `self.prepend(other.drain())`.
+    /// from the other collection to this one, use `self.prepend(other.drain(..))`.
     ///
     /// # Examples
     ///
@@ -1987,6 +1987,19 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// deque.prepend([1, 2, 3]);
     /// assert_eq!(deque, [1, 2, 3, 4, 5, 6]);
     /// ```
+    ///
+    /// Move values between collections like [`append`][VecDeque::append] does but prepend to the front:
+    ///
+    /// ```
+    /// #![feature(deque_extend_front)]
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut deque1 = VecDeque::from([4, 5, 6]);
+    /// let mut deque2 = VecDeque::from([1, 2, 3]);
+    /// deque1.prepend(deque2.drain(..));
+    /// assert_eq!(deque1, [1, 2, 3, 4, 5, 6]);
+    /// assert!(deque2.is_empty());
+    /// ```
     #[unstable(feature = "deque_extend_front", issue = "146975")]
     #[track_caller]
     pub fn prepend<I: IntoIterator<Item = T, IntoIter: DoubleEndedIterator>>(&mut self, other: I) {