Add missing return descriptions for a variety of functions in wp-includes/link-template.php.

Props colorful-tones, dustyf.
See #30406.

Built from https://develop.svn.wordpress.org/trunk@30628


git-svn-id: http://core.svn.wordpress.org/trunk@30618 1a063a9b-81f0-0310-95a4-ce76da25c4cd