Docs: Clarified the purpose of the parent_id query

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

@@ -3,11 +3,7 @@
 
 added[5.0.0]
 
-The `parent_id` query can be used to find a child document pointing to a particular parent id.
+The `parent_id` query can be used to find child documents which belong to a particular parent:
-
-The actual underlying Lucene field that is used to store to what parent id a child document is referring to
-is determined by the child type's `_parent` field. This query helps by selecting the right field based
-on the specified child type. Example:
 
 [source,js]
 --------------------------------------------------
@@ -19,13 +15,29 @@ on the specified child type. Example:
 }
 --------------------------------------------------
 
+The above is functionally equivalent to using the following
+<<query-dsl-has-parent-query, `has_parent`>> query, but performs
+better as it does not need to do a join:
+
+[source,js]
+--------------------------------------------------
+{
+  "has_parent": {
+    "type": "blog",
+    "query": {
+      "term": {
+        "_id": "1"
+      }
+    }
+  }
+}
+--------------------------------------------------
+
 ==== Parameters
 
 This query has two required parameters:
 
 [horizontal]
-`type`::
+`type`::  The **child** type. This must be a type with `_parent` field.
-The child type. This must be a type with `_parent` field.
 
-`id`::
+`id`::    The required parent id select documents must referrer to.
-The required parent id select documents must referrer to.