luapi: graphdb wrapper: annotate type parameter

experimental/uloapi/src/main/java/info/mathhub/uloapi/query/GraphDB.java

@@ -22,6 +22,8 @@ public class GraphDB {
     /**
      * Functional interface that defines a function for running operations on
      * a GraphDB database.
+     *
+     * @param <T> Return type of the operation.
      */
     public interface Operation<T> {
         /**