Enhance lighttpd doc: enable fastcgi check-local

Uh, this issue kept me busy for hours. My lighttpd installation had the `check-local` option of [fastcgi](http://redmine.lighttpd.net/projects/1/wiki/Docs_ModFastCGI) disabled because the PHP processes are run as different users for security reasons (a separate user for each virtual host). However, owncloud uses URLs like `/owncloud/index.php/apps/calendar/js/l10n.php` such that lighttpd takes the full URL as the file to be run (which does not exist). PHP seems to try to fix this and runs `/owncloud/index.php`, but then `PATH_INFO` is empty (it should be `/apps/calendar/js/l10n.php`). owncloud then behaves strangely because all kinds of AJAX calls fail (silently). For example, the configuration cannot be saved.

With `check-local` enabled, lighttpd is able to determine the correct file and sets `PATH_INFO` properly. I hope that this hint in the documentation will save other admins from some hours of unproductive work. ;)