Improve inline documentation for `date_i18n()`, `_http_build_query()`, `wp_checkdate()`, and `wp_auth_check()`.

Props morganestes.
See #26185.

Built from https://develop.svn.wordpress.org/trunk@28917


git-svn-id: http://core.svn.wordpress.org/trunk@28716 1a063a9b-81f0-0310-95a4-ce76da25c4cd