Documentation of use_redirect_parameter_if_present

[rarog]

The behaviour of use_redirect_parameter_if_present doesn't have enough documentation.
I had first to dig through the code to find out, that if the option is enabled, the corresponding parameter via GET should be named "redirect".
I had to further debug with trial and error until I found out, that the redirect parameter may not start with a trailing slash. Especially this should be documented in detail or function routeExists in RedirectCallback.php should do a ltrim($route, '/') to make sure, that a trailing slash is removed and the function is more tolerant.