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: #29569
Fixes: #29439
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/n-api.md

@@ -1774,6 +1774,9 @@ napi_status napi_create_date(napi_env env,
 
 Returns `napi_ok` if the API succeeded.
 
+This API does not observe leap seconds; they are ignored, as
+ECMAScript aligns with POSIX time specification.
+
 This API allocates a JavaScript `Date` object.
 
 JavaScript `Date` objects are described in
@@ -2433,6 +2436,9 @@ napi_status napi_get_date_value(napi_env env,
 * `[out] result`: Time value as a `double` represented as milliseconds
 since midnight at the beginning of 01 January, 1970 UTC.
 
+This API does not observe leap seconds; they are ignored, as
+ECMAScript aligns with POSIX time specification.
+
 Returns `napi_ok` if the API succeeded. If a non-date `napi_value` is passed
 in it returns `napi_date_expected`.