Update rest-api-spec

Author: delvedor

output/schema/validation-errors.json

@@ -68,6 +68,12 @@
         "type": "number",
         "description": "Starting offset (default: 0)"
       },
+      "force_synthetic_source": {
+        "type": "boolean",
+        "description": "Should this request force synthetic _source? Use this to test if the mapping supports synthetic _source and to get a sense of the worst case performance. Fetches with this enabled will be slower the enabling synthetic source natively in the index.",
+        "visibility": "feature_flag",
+        "feature_flag": "es.index_mode_feature_flag_registered"
+      },
       "ignore_unavailable": {
         "type": "boolean",
         "description": "Whether specified concrete indices should be ignored when unavailable (missing or closed)"