[Finder] Add info about possibilities offered by SplFileInfo

Minor documentation improvement
I'm not sure about the final conclusion of your comment, @cordoval .

It appears on the web, but not here (code), which I consider a (mild) flaw because you expect the user will look into the code to get it, or that the user has an internet connection. This is vital information for the user (developer) to be able to use this component properly.

I consider irrelevant the fact that it extends \SplFileInfo or SplFileInfo because I'm referring to http://php.net/splfileinfo at the PHP level (not Symfony-level), but maybe I should put it in the description, indeed.