Update multiple-guards.md

It took me a while to figure out, that you have to add `protected array $guard_name = ['web', 'admin'];` in order to allow an user to use roles / permissions from different guards. Especially since it's not a requirement to specify the `$guard_name`.

docs/basic-usage/multiple-guards.md

@@ -54,7 +54,7 @@ $user->hasPermissionTo('publish articles', 'admin');
 
 ## Assigning permissions and roles to guard users
 
-You can use the same core methods to assign permissions and roles to users; just make sure the `guard_name` on the permission or role matches the guard of the user, otherwise a `GuardDoesNotMatch` or `Role/PermissionDoesNotExist` exception will be thrown.
+You can use the same core methods to assign permissions and roles to users; just make sure the `guard_name` on the permission or role matches the guard of the user, otherwise a `GuardDoesNotMatch` or `Role/PermissionDoesNotExist` exception will be thrown. If your user is able to consume multiple roles or permissions from different guards; make sure the user's `$guard_name` property or `guardName()` method returns all allowed guards in an array.
 
 
 ## Using blade directives with multiple guards