Docs: Parent field is no longer indexed and should use parent_id inst…

…ead of term query

Closes #22517

docs/reference/mapping/fields/parent-field.asciidoc

@@ -59,16 +59,18 @@ See the <<query-dsl-has-child-query,`has_child`>> and
 the <<search-aggregations-bucket-children-aggregation,`children`>> aggregation,
 and <<parent-child-inner-hits,inner hits>> for more information.
 
-The value of the `_parent` field is accessible in queries, aggregations,
-and scripts:
+The value of the `_parent` field is accessible in aggregations
+and scripts, and may be queried with the
+<<query-dsl-parent-id-query, `parent_id` query>>:
 
 [source,js]
 --------------------------
 GET my_index/_search
 {
   "query": {
-    "terms": {
-      "_parent": [ "1" ] <1>
+    "parent_id": { <1>
+      "type": "my_parent",
+      "id": "1"
     }
   },
   "aggs": {
@@ -92,7 +94,7 @@ GET my_index/_search
 // CONSOLE
 // TEST[continued]
 
-<1> Querying on the `_parent` field (also see the <<query-dsl-has-parent-query,`has_parent` query>> and the <<query-dsl-has-child-query,`has_child` query>>)
+<1> Querying the id of the `_parent` field (also see the <<query-dsl-has-parent-query,`has_parent` query>> and the <<query-dsl-has-child-query,`has_child` query>>)
 <2> Aggregating on the `_parent` field (also see the <<search-aggregations-bucket-children-aggregation,`children`>> aggregation)
 <3> Accessing the `_parent` field in scripts