changed description for update by query (issue #315)

Author: l-trotta

specification/_json_spec/update_by_query.json

@@ -2,7 +2,7 @@
   "update_by_query": {
     "documentation": {
       "url": "https://www.elastic.co/guide/en/elasticsearch/reference/master/docs-update-by-query.html",
-      "description": "Performs an update on every document in the index without changing the source,\nfor example to pick up a mapping change."
+      "description": "Updates documents that match the specified query. If no query is specified,\n performs an update on every document in the index without changing the source,\nfor example to pick up a mapping change."
     },
     "stability": "stable",
     "visibility": "public",