[DOCS] Adds missing add default field API (#86332)

docs/api/upgrade-assistant.asciidoc

@@ -22,5 +22,6 @@ include::upgrade-assistant/status.asciidoc[]
 include::upgrade-assistant/reindexing.asciidoc[]
 include::upgrade-assistant/batch_reindexing.asciidoc[]
 include::upgrade-assistant/batch_queue.asciidoc[]
+include::upgrade-assistant/default-field.asciidoc[]
 include::upgrade-assistant/check_reindex_status.asciidoc[]
 include::upgrade-assistant/cancel_reindex.asciidoc[]

docs/api/upgrade-assistant/check_reindex_status.asciidoc

@@ -6,8 +6,6 @@
 
 experimental[] Check the status of the reindex operation.
 
-Check the status of the reindex operation.
-
 [[check-reindex-status-request]]
 ==== Request
 

docs/api/upgrade-assistant/default-field.asciidoc

@@ -0,0 +1,113 @@
+[[upgrade-assistant-api-default-field]]
+=== Add default field API
+++++
+<titleabbrev>Add default field</titleabbrev>
+++++
+
+experimental[] In {es} 7.0 and later, some query types, such as Simple Query String, have a limit to the number of fields they can query against. 
+To configure the cap in {es}, set the `indices.query.bool.max_clause_count` cluster setting, which is 1024 by default.
+
+For indices with more fields than the cap, add the `index.query.default_field` index setting to inform {es} which 
+fields to use by default when no field is specified for a query. Use the add default field API to add the `index.query.default_field` setting to an {es} index.
+
+[[upgrade-assistant-api-default-field-request]]
+==== Request
+
+To add the `index.query.default_field` setting to an {es} index, submit a POST request to `/api/upgrade_assistant/add_query_default_field/<index>`:
+
+[source,js]
+--------------------------------------------------
+GET /api/upgrade_assistant/add_query_default_field/myIndex
+{
+  "fieldTypes": ["text", "keyword"], <1>
+  "otherFields": ["myField.*"] <2>
+}
+--------------------------------------------------
+// KIBANA
+
+<1> A required array of {es} field types that generate the list of fields.
+<2> An optional array of additional field names, dot-deliminated.
+
+To add the `index.query.default_field` index setting to the specified index, {kib} generates an array of all fields from the index mapping.
+The fields contain the types specified in `fieldTypes`. {kib} appends any other fields specified in `otherFields` to the array of default fields.
+
+[[upgrade-assistant-api-default-field-response-codes]]
+==== Response codes
+
+`200`::
+  Indicates a successful call.
+
+ `400`::
+  Indicates that the index already has the `index.query.default_field` setting. No changes are made to the index. 
+
+[[upgrade-assistant-api-default-field-response-body]]
+==== Response body
+
+The response body contains a JSON structure, similar to the following:
+
+[source,js]
+--------------------------------------------------
+{
+  "acknowledged": true
+}
+--------------------------------------------------
+
+[[upgrade-assistant-api-default-field-example]]
+==== Example
+
+Your index contains following mappings:
+
+[source,js]
+--------------------------------------------------
+GET /myIndex/_mappings
+{
+  "myIndex": {
+    "mappings": {
+      "properties": {
+        "field1": { "type": "text" },
+        "field2": { "type": "float" },
+        "nestedfield": {
+          "properties": {
+            "field3": { "type": "keyword" },
+            "field4": { "type": "long" },
+          }
+        }
+      }
+    }
+  }
+}
+--------------------------------------------------
+// CONSOLE
+
+Make the following request to {kib}:
+
+[source,js]
+--------------------------------------------------
+GET /api/upgrade_assistant/add_query_default_field/myIndex
+{
+  "fieldTypes": ["text", "long"],
+  "otherFields": ["field2"]
+}
+--------------------------------------------------
+// KIBANA
+
+The API returns the following:
+
+[source,js]
+--------------------------------------------------
+GET /myIndex/_settings?flat_settings=true
+{
+  "myIndex": {
+    "settings": {
+      "index.query.default_field": [
+        "field1",
+        "nestedfield.field4",
+        "field2",
+      ]
+    }
+  }
+}
+--------------------------------------------------
+// CONSOLE
+
+{kib} generates the `field1` and `nestedfield.field4` values based on the specified `fieldTypes`, then appends the `otherFields` to the array.