doc: refine napi_get_property_names() doc

Document that only enumerable, string-keyed properties are returned.

Fixes: https://github.com/nodejs/abi-stable-node/issues/307
PR-URL: https://github.com/nodejs/node/pull/20427


Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>