doc: add experimental flag

Co-authored-by: Chengzhong Wu <[email protected]>

Author: miguelmarcondesf

doc/api/n-api.md

@@ -2641,9 +2641,10 @@ ECMAScript Language Specification.
 
 <!-- YAML
 added: REPLACEME
-napiVersion: 8
 -->
 
+> Stability: 1 - Experimental
+
 ```cpp
 napi_status napi_create_object_with_properties(napi_env env,
                                                napi_value prototype_or_null,