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.

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git-svn-id: http://core.svn.wordpress.org/trunk@29628 1a063a9b-81f0-0310-95a4-ce76da25c4cd