[Backport 2.14] Add documentation for constant_keyword field type

_field-types/supported-field-types/constant-keyword.md

@@ -0,0 +1,41 @@
+---
+layout: default
+title: Constant keyword
+nav_order: 71
+has_children: false
+parent: String field types
+grand_parent: Supported field types
+---
+
+# Constant keyword field type
+
+A constant keyword field uses the same value for all documents in the index. 
+
+When a search request spans multiple indexes, you can filter on a constant keyword field to match documents from indexes with the given constant value but not from indexes with a different value.
+
+## Example
+
+The following query creates a mapping with a constant keyword field:
+
+```json
+PUT romcom_movies
+{
+  "mappings" : {
+    "properties" : {
+      "genre" : {
+        "value" : "Romantic comedy"
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+## Parameters
+
+The following table lists the parameters accepted by constant keyword field types. All values are required.
+
+Parameter | Description 
+:--- | :--- 
+`value` | The string field value for all documents in the index.
+

_field-types/supported-field-types/string.md

@@ -20,3 +20,4 @@ Field data type | Description
 [`text`]({{site.url}}{{site.baseurl}}/opensearch/supported-field-types/text/) | A string that is analyzed. Useful for full-text search.
 [`match_only_text`]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/match-only-text/) | A space-optimized version of a `text` field.
 [`token_count`]({{site.url}}{{site.baseurl}}/opensearch/supported-field-types/token-count/)  | Counts the number of tokens in a string. 
+[`constant_keyword`]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/constant-keyword/)  | Similar to `keyword` but uses a single value for all documents.