support description on builtin 'post' post-type

Adds a conditional to use the correct add_submenu_page() syntax when adding a "Description" menu link to the "Posts" menu.

For what appear to be historical reasons, the builtin 'post' post-type does't use the same slug syntax as custom post types in `add_submenu_page()`. For more information and a full list of builtin admin menu slugs, see [the WordPress documentation on `add_submenu_page()`](http://codex.wordpress.org/Function_Reference/add_submenu_page).