[API] Added the `terminate_after` parameter to the REST spec for "Count" API

Closes #26895

rest-api-spec/src/main/resources/rest-api-spec/api/count.json

@@ -67,6 +67,10 @@
         "lenient": {
           "type" : "boolean",
           "description" : "Specify whether format-based query failures (such as providing text to a numeric field) should be ignored"
+        },
+        "terminate_after" : {
+          "type" : "number",
+          "description" : "The maximum count for each shard, upon reaching which the query execution will terminate early"
         }
       }
     },