doc: mark napi_add_finalizer experimental

mhdawson · mhdawson · commit 98954fd5489f · 2018-11-22T18:47:40.000Z
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
diff --git a/doc/api/n-api.md b/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
