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: https://github.com/nodejs/node/issues/24509
PR-URL: https://github.com/nodejs/node/pull/24572


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>