Add missing comments in StateStoreService (#5959)

hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/store/StateStoreService.java

@@ -64,6 +64,12 @@ import org.apache.hadoop.classification.VisibleForTesting;
  * <li>File {@link
  * org.apache.hadoop.hdfs.server.federation.store.driver.impl.StateStoreFileImpl
  * StateStoreFileImpl}
+ * <li>FileSystem {@link
+ * org.apache.hadoop.hdfs.server.federation.store.driver.impl.StateStoreFileSystemImpl
+ * StateStoreFileSystemImpl}
+ * <li>MySQL {@link
+ * org.apache.hadoop.hdfs.server.federation.store.driver.impl.StateStoreMySQLImpl
+ * StateStoreMySQLImpl}
  * <li>ZooKeeper {@link
  * org.apache.hadoop.hdfs.server.federation.store.driver.impl.StateStoreZooKeeperImpl
  * StateStoreZooKeeperImpl}