Update nested-query.asciidoc

Added note that fields inside a nested query must be full qualified.

docs/reference/query-dsl/queries/nested-query.asciidoc

@@ -47,7 +47,9 @@ And here is a sample nested query usage:
 
 The query `path` points to the nested object path, and the `query` (or
 `filter`) includes the query that will run on the nested docs matching
-the direct path, and joining with the root parent docs.
+the direct path, and joining with the root parent docs. Note that any
+fields referenced inside the query must use the complete path (fully 
+qualified).
 
 The `score_mode` allows to set how inner children matching affects
 scoring of parent. It defaults to `avg`, but can be `total`, `max` and