doc: mark some node-api functions as experimental

PR-URL: nodejs/node#42987
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>

doc/api/n-api.md

@@ -1185,6 +1185,8 @@ This API throws a JavaScript `RangeError` with the text provided.
 added: v16.14.0
 -->
 
+> Stability: 1 - Experimental
+
 ````c
 NAPI_EXTERN napi_status node_api_throw_syntax_error(napi_env env,
                                                     const char* code,
@@ -1302,6 +1304,8 @@ This API returns a JavaScript `RangeError` with the text provided.
 added: v16.14.0
 -->
 
+> Stability: 1 - Experimental
+
 ```c
 NAPI_EXTERN napi_status node_api_create_syntax_error(napi_env env,
                                                      napi_value code,