Clarify the spec wrt `useOldISOCodes` system property

Hebrew/Indonesian/Yiddish ISO 639 language codes failed to work in backward compatible manner even if system property java.locale.useOldISOCodes sets to true.

Code snippet:
import java.util.Locale;
public class Test1 {
    public static void main(String[] args) {
        System.setProperty("java.locale.useOldISOCodes","true");
        new Test1().test();
    }
    String languages[] = new String[]{"he", "yi", "id"};
    String excepted[] = new String[]{"iw", "ji", "in"};
    private void test() {
        Locale locale = null;
        for (int i = 0; i < languages.length; i++) {
            locale = new Locale(languages[i]);
            if (excepted[i] != null) {
                if (!locale.getLanguage().equals(excepted[i])) {
                    System.out.println(
                            "getLanguage returned wrong value : "
                                    + locale.getLanguage()
                                    + " language = " + excepted[i]
                    );
                }
            }
        }
    }
}