Update File.php

Adding warning, took me a couple hours to figure this out...

The file does not get moved because of this line:
https://github.com/silverstripe/silverstripe-framework/blob/3.1/filesystem/File.php#L515

Maybe add to the method doc, telling people to set ParentId and Name instead if they want to change the location of a file (if that's the correct method).
Merge pull request #2899 from IgorNadj/patch-1

Update File.php