Clarify inline documentation for the `_is_valid_nav_menu_item()` and `wp_get_nav_menu_items()` in wp-includes/nav-menu.php.

Props morganestes for the initial patch.
Fixes #29914.


git-svn-id: https://develop.svn.wordpress.org/trunk@29868 602fd350-edb4-49c9-b593-d223f7449a82