Document why HybridJacksonPool has to be public

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

@@ -29,6 +29,8 @@
  * recalculating the position of that bucket also during the release. The heads of the linked lists are hold in an
  * {@link AtomicReferenceArray} where each head has a distance of 16 positions from the adjacent ones to prevent
  * the false sharing problem.
+ * <p>
+ * This class is public in order to allow its reuse also in other projects. 
  */
 public class HybridJacksonPool implements RecyclerPool<BufferRecycler> {