doc: add leap second behavior notes for napi methods

napi_create_date and napi_get_date_value ignore leap seconds as per
ECMAScript spec that follows POSIX spec for time, comments added to the
documentation where added fo clarify it.

PR-URL: https://github.com/nodejs/node/pull/29569
Fixes: https://github.com/nodejs/node/issues/29439


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>