doc: mark napi_add_finalizer experimental

This was missed when the function was added. As it
is experimental in the code it should also be
experimental in the docs.

Fixes: nodejs#24509
PR-URL: nodejs#24572
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/n-api.md

@@ -3799,6 +3799,9 @@ callback was associated with the wrapping, it will no longer be called when the
 JavaScript object becomes garbage-collected.
 
 ### napi_add_finalizer
+
+> Stability: 1 - Experimental
+
 <!-- YAML
 added: v8.0.0
 napiVersion: 1