add comment for XorShiftThreadProbe

src/main/java/io/vertx/core/json/jackson/HybridJacksonPool.java

@@ -197,6 +197,11 @@ public boolean test(Thread thread) {
     }
   }
 
+  /**
+   * This class is used to hash the thread requiring a pooled resource using a multiplicative
+   * Fibonacci hashing implementation. The resulting hash is then used to calculate the
+   * index of the bucket in the pool from where the pooled resource has to be retrieved.
+   */
   private static class XorShiftThreadProbe {
 
     private final int mask;