doc: refine napi_get_property_names() doc

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

Fixes: nodejs/abi-stable-node#307
PR-URL: #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>

doc/api/n-api.md

@@ -2520,7 +2520,9 @@ and [`napi_get_element`][].
 
 Returns `napi_ok` if the API succeeded.
 
-This API returns the array of properties for the `Object` passed in.
+This API returns the names of the enumerable properties of `object` as an array
+of strings. The properties of `object` whose key is a symbol will not be
+included.
 
 #### napi_set_property
 <!-- YAML