Remove HTML `<head>` tags from DocBlock summaries in wp-admin/admin-header.php.

Also better-specify the `$hook_suffix` value in the `admin_head-$hook_suffix` hook docs per hook documentation precedent.

Props rarst for the initial patch.
See #30473


git-svn-id: https://develop.svn.wordpress.org/trunk@30535 602fd350-edb4-49c9-b593-d223f7449a82