Where is saved the value of the 'help text' of field of content type? I know that you can set help text for any field in the content type, but where this text is saved in databese, because I want to access it in other way not only for tooltip?

Thanks
It's in the field instance settings (stored in the field_config_instance table). The preferred API method to use is field_info_instance():

$info = field_info_instance('node', 'field_foo', 'article');
$help_text = $info['description'];