docs: added p/c sort example

Author: martijnvg

docs/reference/query-dsl/has-parent-query.asciidoc

@@ -47,3 +47,32 @@ matching parent document. The score mode can be specified with the
     }
 }
 --------------------------------------------------
+
+[float]
+==== Sorting
+
+Child documents can't be sorted by fields in matching parent documents via the
+regular sort options. If you need to sort child documents by field in the parent
+documents then you can should use the `function_score` query and then just sort
+by `_score`.
+
+Sorting tags by parent document' `view_count` field:
+
+[source,js]
+--------------------------------------------------
+{
+    "query": {
+        "has_parent" : {
+            "parent_type" : "blog",
+            "score_mode" : "score",
+            "query" : {
+                "function_score" : {
+                    "script_score": {
+                        "script": "_score * doc['view_count'].value"
+                    }
+                }
+            }
+        }
+    }
+}
+--------------------------------------------------