Docs: route_to() logic

[MGatner]

The User Guide has examples of using route_to() to return a URI path, which will fail in certain circumstances (e.g. subfolders). Docs should be updated to be clear that this returns a route, and if a URI is needed then it should be passed through site_url() (or just use url_to()).

See also: codeigniter4/shield#267 (comment)