Add comments to Linked List code.

Author: trekhleb

src/data-structures/linked-list/LinkedList.js

@@ -128,13 +128,17 @@ export default class LinkedList {
    */
   deleteTail() {
     const deletedTail = this.tail;
+
     if (this.head === this.tail) {
       // There is only one node in linked list.
       this.head = null;
       this.tail = null;
 
       return deletedTail;
     }
+
+    // If there are many nodes in linked list...
+
     // Rewind to the last node and delete "next" link for the node before the last one.
     let currentNode = this.head;
     while (currentNode.next) {