[DOCS] Doc'ed cluster pending tasks

docs/reference/cluster.asciidoc

@@ -33,6 +33,8 @@ include::cluster/health.asciidoc[]
 
 include::cluster/state.asciidoc[]
 
+include::cluster/pending.asciidoc[]
+
 include::cluster/reroute.asciidoc[]
 
 include::cluster/update-settings.asciidoc[]

docs/reference/cluster/pending.asciidoc

@@ -0,0 +1,44 @@
+[[cluster-pending]]
+== Pending cluster tasks
+
+The pending cluster tasks API returns a list of any cluster-level changes
+(e.g. create index, update mapping, allocate or fail shard) which have not yet
+been executed.
+
+[source,js]
+--------------------------------------------------
+$ curl -XGET 'http://localhost:9200/_cluster/pending_tasks'
+--------------------------------------------------
+
+Usually this will return an empty list as cluster-level changes are usually
+fast. However if there are tasks queued up, the output will look something
+like this:
+
+[source,js]
+--------------------------------------------------
+{
+   "tasks": [
+      {
+         "insert_order": 101,
+         "priority": "URGENT",
+         "source": "create-index [foo_9], cause [api]",
+         "time_in_queue_millis": 86,
+         "time_in_queue": "86ms"
+      },
+      {
+         "insert_order": 46,
+         "priority": "HIGH",
+         "source": "shard-started ([foo_2][1], node[tMTocMvQQgGCkj7QDHl3OA], [P], s[INITIALIZING]), reason [after recovery from gateway]",
+         "time_in_queue_millis": 842,
+         "time_in_queue": "842ms"
+      },
+      {
+         "insert_order": 45,
+         "priority": "HIGH",
+         "source": "shard-started ([foo_2][0], node[tMTocMvQQgGCkj7QDHl3OA], [P], s[INITIALIZING]), reason [after recovery from gateway]",
+         "time_in_queue_millis": 858,
+         "time_in_queue": "858ms"
+      }
+  ]
+}
+--------------------------------------------------