Improve the `@param` docs for `trackback_response()` and `validate_another_blog_signup()`.

See #30224.

Built from https://develop.svn.wordpress.org/trunk@30662


git-svn-id: http://core.svn.wordpress.org/trunk@30652 1a063a9b-81f0-0310-95a4-ce76da25c4cd