Clarify that VecDeque::swap can panic

The previous documentation mentioned this, but ambiguously used the term "fail".

This clarifies that the function will panic if the index is out of bounds, instead of silently failing and not doing anything.

src/liballoc/vec_deque.rs

@@ -459,10 +459,12 @@ impl<T> VecDeque<T> {
     ///
     /// `i` and `j` may be equal.
     ///
-    /// Fails if there is no element with either index.
-    ///
     /// Element at index 0 is the front of the queue.
     ///
+    /// # Panics
+    ///
+    /// Panics if either index is out of bounds.
+    ///
     /// # Examples
     ///
     /// ```