Add description to org.elasticsearch.tasks package (elastic#19700)

Yet more readable docs!

core/src/main/java/org/elasticsearch/tasks/package-info.java

@@ -0,0 +1,24 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/**
+ * Support for viewing and modifying in flight actions ({@link org.elasticsearch.tasks.Task}s) and saving their results to an index. This
+ * includes getting detailed descriptions and canceling tasks that support it.
+ */
+package org.elasticsearch.tasks;