Document time of back operations of a Linked List

Popping and pushing from the end of a linked list is constant time. This
documentation is already there for popping and pushing from the front.

@bors: r+ 38fe8d2 rollup

src/liballoc/collections/linked_list.rs

@@ -627,7 +627,9 @@ pub fn pop_front(&mut self) -> Option<T> {
         self.pop_front_node().map(Node::into_element)
     }
 
-    /// Appends an element to the back of a list
+    /// Appends an element to the back of a list.
+    ///
+    /// This operation should compute in O(1) time.
     ///
     /// # Examples
     ///
@@ -647,6 +649,8 @@ pub fn push_back(&mut self, elt: T) {
     /// Removes the last element from a list and returns it, or `None` if
     /// it is empty.
     ///
+    /// This operation should compute in O(1) time.
+    ///
     /// # Examples
     ///
     /// ```