Update VecDeque documentation to specify direction of index 0 (#34920)

Author: abhijeetbhagat

src/libcollections/vec_deque.rs

@@ -402,6 +402,8 @@ impl<T> VecDeque<T> {
 
     /// Retrieves an element in the `VecDeque` by index.
     ///
+    /// Element at index 0 is the front of the queue.
+    ///
     /// # Examples
     ///
     /// ```
@@ -425,6 +427,8 @@ impl<T> VecDeque<T> {
 
     /// Retrieves an element in the `VecDeque` mutably by index.
     ///
+    /// Element at index 0 is the front of the queue.
+    ///
     /// # Examples
     ///
     /// ```
@@ -456,6 +460,8 @@ impl<T> VecDeque<T> {
     ///
     /// Fails if there is no element with either index.
     ///
+    /// Element at index 0 is the front of the queue.
+    ///
     /// # Examples
     ///
     /// ```
@@ -1180,6 +1186,8 @@ impl<T> VecDeque<T> {
     ///
     /// Returns `None` if `index` is out of bounds.
     ///
+    /// Element at index 0 is the front of the queue.
+    ///
     /// # Examples
     ///
     /// ```
@@ -1214,6 +1222,8 @@ impl<T> VecDeque<T> {
     ///
     /// Returns `None` if `index` is out of bounds.
     ///
+    /// Element at index 0 is the front of the queue.
+    ///
     /// # Examples
     ///
     /// ```
@@ -1245,6 +1255,8 @@ impl<T> VecDeque<T> {
     /// end is closer to the insertion point will be moved to make room,
     /// and all the affected elements will be moved to new positions.
     ///
+    /// Element at index 0 is the front of the queue.
+    ///
     /// # Panics
     ///
     /// Panics if `index` is greater than `VecDeque`'s length
@@ -1472,6 +1484,8 @@ impl<T> VecDeque<T> {
     /// room, and all the affected elements will be moved to new positions.
     /// Returns `None` if `index` is out of bounds.
     ///
+    /// Element at index 0 is the front of the queue.
+    ///
     /// # Examples
     ///
     /// ```
@@ -1651,6 +1665,8 @@ impl<T> VecDeque<T> {
     ///
     /// Note that the capacity of `self` does not change.
     ///
+    /// Element at index 0 is the front of the queue.
+    ///
     /// # Panics
     ///
     /// Panics if `at > len`