Add comment to Dropper

src/liballoc/collections/vec_deque.rs

@@ -144,6 +144,8 @@ impl<T: Clone> Clone for VecDeque<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<#[may_dangle] T> Drop for VecDeque<T> {
     fn drop(&mut self) {
+        /// Runs the destructor for all items in the slice when it gets dropped (normally or
+        /// during unwinding).
         struct Dropper<'a, T>(&'a mut [T]);
 
         impl<'a, T> Drop for Dropper<'a, T> {