[5.7] Changed description for authenticate method (#25649)

* [5.7] Changed description for `authenticate` method
 - Changed description for authenticate method, since check method has the same description.

* Update GuardHelpers.php

GuardHelpers.php

@@ -25,7 +25,7 @@ trait GuardHelpers
     protected $provider;
 
     /**
-     * Determine if the current user is authenticated.
+     * Determine if current user is authenticated. If not, throw an exception.
      *
      * @return \Illuminate\Contracts\Auth\Authenticatable
      *