Add missing index query parameter to msearch (#4980)

Author: pquentin

output/openapi/elasticsearch-openapi.json

@@ -1,11 +1,4 @@
 {
-  "endpointErrors": {
-    "msearch": {
-      "request": [
-        "Request: missing json spec query parameter 'index'"
-      ],
-      "response": []
-    }
-  },
+  "endpointErrors": {},
   "generalErrors": []
 }

output/openapi/elasticsearch-serverless-openapi.json

@@ -99,6 +99,10 @@ export interface Request extends RequestBase {
      * @server_default false
      */
     include_named_queries_score?: boolean
+    /**
+     * Comma-separated list of data streams, indices, and index aliases to use as default
+     */
+    index?: Indices
     /**
      * Maximum number of concurrent searches the multi search API can execute.
      * Defaults to `max(1, (# of data nodes * min(search thread pool size, 10)))`.