Adding priority list and executing description to the pending tasks doc (#74456) (#78259)

* Adding priority to the pending tasks doc

https://github.com/elastic/elasticsearch/pull/19448#discussion_r70969307
https://github.com/elastic/elasticsearch/blob/917fea7c5d0ea31ae1ccf301298f5f006216bb9f/core/src/main/java/org/elasticsearch/common/Priority.java#L29

* Adding executing into the cluster pending tasks

* Update docs/reference/cluster/pending.asciidoc

Co-authored-by: Henning Andersen <33268011+henningandersen@users.noreply.github.com>

Co-authored-by: Henning Andersen <33268011+henningandersen@users.noreply.github.com>

Co-authored-by: Leaf-Lin <39002973+Leaf-Lin@users.noreply.github.com>
Co-authored-by: Henning Andersen <33268011+henningandersen@users.noreply.github.com>

docs/reference/cluster/pending.asciidoc

@@ -50,12 +50,15 @@ include::{es-repo-dir}/rest-api/common-parms.asciidoc[tag=master-timeout]
     the task queue.
     
 `priority`::
-    (string) The priority of the pending task.
+    (string) The priority of the pending task. The valid priorities in descending priority order are: `IMMEDIATE` > `URGENT` > `HIGH` > `NORMAL` > `LOW` > `LANGUID`. 
     
 `source`::
     (string) A general description of the cluster task that may include a reason 
     and origin. 
     
+`executing`::
+    (boolean) True or false, indicating whether the pending tasks is currently getting executed or not.
+    
 `time_in_queue_millis`::
     (integer) The time expressed in milliseconds since the task is waiting for 
     being performed.
@@ -78,6 +81,7 @@ However, if there are tasks queued up, the response will look similar like this:
          "insert_order": 101,
          "priority": "URGENT",
          "source": "create-index [foo_9], cause [api]",
+         "executing" : true,
          "time_in_queue_millis": 86,
          "time_in_queue": "86ms"
       },
@@ -85,6 +89,7 @@ However, if there are tasks queued up, the response will look similar like this:
          "insert_order": 46,
          "priority": "HIGH",
          "source": "shard-started ([foo_2][1], node[tMTocMvQQgGCkj7QDHl3OA], [P], s[INITIALIZING]), reason [after recovery from shard_store]",
+         "executing" : false,
          "time_in_queue_millis": 842,
          "time_in_queue": "842ms"
       },
@@ -92,6 +97,7 @@ However, if there are tasks queued up, the response will look similar like this:
          "insert_order": 45,
          "priority": "HIGH",
          "source": "shard-started ([foo_2][0], node[tMTocMvQQgGCkj7QDHl3OA], [P], s[INITIALIZING]), reason [after recovery from shard_store]",
+         "executing" : false,
          "time_in_queue_millis": 858,
          "time_in_queue": "858ms"
       }