Document that `napi_get_date_value()` ignores leap seconds as per spec

[gabrielschulhof]

#29401 (comment):

I'm actually not sure whether it does or does not account for leap seconds.

From https://tc39.es/ecma262/#sec-time-values-and-time-range:

Time in ECMAScript does not observe leap seconds; they are ignored.

[levhita]

I'm guessing is the same case with napi_create_date

[levhita]

Here, finished PR #29569