Document that wildcard % doesn't work as first argument in menu paths

When you set up a menu callback, if you enter the wildcard at first argument (to delegate authentication to a module for instance):

  $items['%/authenticate'] = array(
    'page callback' => 'oauth_delegate',
    'page arguments' => array(0, 1),
    'access callback' => 'oauth_implements',
    'access arguments' => array(0),
    'type' => MENU_CALLBACK,
    'file' => 'oauth.inc',
  );

/**
 * Check if a module implementing oauth exists.
 */
function oauth_implements($module) {
  return function_exists($module . '_oauth_info');
}

Then it doesn't work. The function leads to 404 Page not found and the menu router doesn't even visit the access callback.

To fix the problem i added a fixed first argument:

  $items['oauth/%/authenticate'] = array(
    'page callback' => 'oauth_delegate',
    'page arguments' => array(1, 2),
    'access callback' => 'oauth_implements',
    'access arguments' => array(1),
    'type' => MENU_CALLBACK,
    'file' => 'oauth.inc',
  );

But for patterns like delegation, i.e. OAuth based authentication where module implements and instantiate the protocol, it is nice for a user perspective to have urls like:

"twitter/authenticate"
"facebook/authenticate"
"linkedin/authenticate"