Added note about aliased typehints failing to autoload

Related [laravel/framework issue #1861](https://github.com/laravel/framework/issues/1861): Although PHP's reasoning for not autoloading undefined type-hinted classes is solid (undefined classes clearly can't have instances and therefore the type hint raises an error), it's not intuitive in the context of Laravel's IoC. It seems like Laravel should be able to alias all typehints, but since it can't, I think it is important to note that in the docs with the short bit about aliases.