Documented singleton double check idiom, explaining the dynamics that happen on each step for better understanding. Did this due to a PR iluwatar#475

Author: npathai

singleton/src/main/java/com/iluwatar/singleton/ThreadSafeDoubleCheckLocking.java

@@ -53,11 +53,20 @@ private ThreadSafeDoubleCheckLocking() {
   public static ThreadSafeDoubleCheckLocking getInstance() {
     // local variable increases performance by 25 percent
     // Joshua Bloch "Effective Java, Second Edition", p. 283-284
+    
     ThreadSafeDoubleCheckLocking result = instance;
+    // Check if singleton instance is initialized. If it is initialized then we can return the instance.
     if (result == null) {
+      // It is not initialized but we cannot be sure because some other thread might have initialized it
+      // in the meanwhile. So to make sure we need to lock on an object to get mutual exclusion.
       synchronized (ThreadSafeDoubleCheckLocking.class) {
+        // Again assign the instance to local variable to check if it was initialized by some other thread
+        // while current thread was blocked to enter the locked zone. If it was initialized then we can 
+        // return the previously created instance just like the previous null check.
         result = instance;
         if (result == null) {
+          // The instance is still not initialized so we can safely (no other thread can enter this zone)
+          // create an instance and make it our singleton instance.
           instance = result = new ThreadSafeDoubleCheckLocking();
         }
       }