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
Check attachments as well as the current post type when checking for duplicate post slugs. This avoids creating a page with a slug which conflicts with an existing attachment (the inverse is already handled).

Updates the existing test for pages which should take priority over attachments in cases where an existing clash exists.

Fixes #18962
Props boonebgorges

Built from https://develop.svn.wordpress.org/trunk@30629


git-svn-id: http://core.svn.wordpress.org/trunk@30619 1a063a9b-81f0-0310-95a4-ce76da25c4cd